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‎0042.trapping-rain-water/README.md

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# [42. Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/)

‎0042.trapping-rain-water/TrappingRainWater.java

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class TrappingRainWater {
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public int trap(int[] height) {
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int left = 0, right = height.length - 1;
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int res = 0;
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int left_max = 0, right_max = 0;
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while (left < right) {
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if (height[left] < height[right]) {
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if (height[left] >= left_max) {
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left_max = height[left];
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} else {
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res += (left_max - height[left]);
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}
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left++;
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} else {
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if (height[right] >= right_max) {
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right_max = height[right];
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} else {
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res += (right_max - height[right]);
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}
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right--;
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}
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}
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return res;
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}
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}

‎0043.multiply-strings/MultiplyStrings.java

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import java.util.ArrayList;
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class MultiplyStrings {
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private ArrayList<Integer> addStrings(ArrayList<Integer> num1, ArrayList<Integer> num2) {
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ArrayList<Integer> res = new ArrayList<>();
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int carry = 0;
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for (int i = 0; i < num1.size() || i < num2.size(); i++) {
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int digit1 = i < num1.size() ? num1.get(i) : 0;
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int digit2 = i < num2.size() ? num2.get(i) : 0;
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int sum = digit1 + digit2 + carry;
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carry = sum / 10;
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res.add(sum % 10);
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}
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if (carry != 0) {
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res.add(carry);
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}
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return res;
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}
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private ArrayList<Integer> multiplyOneDigit(StringBuilder firstNumber, char secondNumberDigit, int numZeros) {
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ArrayList<Integer> currentResult = new ArrayList<>();
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for (int i = 0; i < numZeros; i++) currentResult.add(0);
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int carry = 0;
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for (int i = 0; i < firstNumber.length(); i++) {
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char firstNumberDigit = firstNumber.charAt(i);
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int multiplication = (secondNumberDigit - '0') * (firstNumberDigit - '0') + carry;
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carry = multiplication / 10;
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currentResult.add(multiplication % 10);
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}
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if (carry != 0) {
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currentResult.add(carry);
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}
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return currentResult;
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}
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public String multiply(String num1, String num2) {
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if (num1.equals("0") || num2.equals("0")) return "0";
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StringBuilder firstNumber = new StringBuilder(num1);
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StringBuilder secondNumber = new StringBuilder(num2);
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firstNumber.reverse();
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secondNumber.reverse();
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int n = firstNumber.length() + secondNumber.length();
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ArrayList<Integer> res = new ArrayList<>();
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for (int i = 0; i < n; i++) res.add(0);
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for (int i = 0; i < secondNumber.length(); i++) {
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res = addStrings(multiplyOneDigit(firstNumber, secondNumber.charAt(i), i), res);
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}
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if (res.get(res.size() - 1) == 0) {
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res.remove(res.size() - 1);
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}
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StringBuilder result = new StringBuilder();
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for (int i = res.size() - 1; i >= 0; i--) {
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result.append(res.get(i));
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}
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return result.toString();
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}
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}

‎0043.multiply-strings/README.md

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# [43. Multiply Strings](https://leetcode.com/problems/multiply-strings/)

‎0045.jump-game-ii/JumpGameIi.java

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class JumpGameIi {
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public int jump(int[] nums) {
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int n = nums.length;
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int maxPosition = 0, end = 0, steps = 0;
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for (int i = 0; i < n - 1; i++) {
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maxPosition = Math.max(maxPosition, i + nums[i]);
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if (i == end) {
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end = maxPosition;
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steps++;
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}
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}
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return steps;
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}
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}

‎0045.jump-game-ii/README.md

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# [45. Jump Game II](https://leetcode.com/problems/jump-game-ii/)
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Complexity Analysis:
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- Time Complexity: $O(n)$. $n$ is the length of `nums`.
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- Space Complexity: $O(1)$.

‎0046.permutations/Permutations.java

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import java.util.ArrayList;
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import java.util.List;
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class Permutations {
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private void backtrack(List<List<Integer>> lists, List<Integer> list, int[] nums) {
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if (list.size() == nums.length) {
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lists.add(new ArrayList<>(list));
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} else {
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for (int num : nums) {
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if (list.contains(num)) continue;
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list.add(num);
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backtrack(lists, list, nums);
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list.remove(list.size() - 1);
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}
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}
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}
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public List<List<Integer>> permute(int[] nums) {
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List<List<Integer>> res = new ArrayList<>();
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backtrack(res, new ArrayList<>(), nums);
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return res;
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}
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}

‎0046.permutations/README.md

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# [46. Permutations](https://leetcode.com/problems/permutations/)

‎0047.permutations-ii/PermutationsIi.java

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import java.util.*;
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class PermutationsIi {
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private void backtrack(LinkedList<Integer> comb, int n, HashMap<Integer, Integer> counter, List<List<Integer>> res) {
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if (comb.size() == n) {
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res.add(new ArrayList<>(comb));
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return;
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}
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for (Map.Entry<Integer, Integer> entry : counter.entrySet()) {
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int num = entry.getKey();
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int count = entry.getValue();
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if (count == 0) continue;
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comb.addLast(num);
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counter.put(num, count - 1);
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backtrack(comb, n, counter, res);
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comb.removeLast();
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counter.put(num, count);
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}
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}
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public List<List<Integer>> permuteUnique(int[] nums) {
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List<List<Integer>> res = new ArrayList<>();
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HashMap<Integer, Integer> counter = new HashMap<>();
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for (int num : nums) {
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counter.put(num, counter.getOrDefault(num, 0) + 1);
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}
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LinkedList<Integer> comb = new LinkedList<>();
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this.backtrack(comb, nums.length, counter, res);
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return res;
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}
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}

‎0047.permutations-ii/README.md

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# [47. Permutations II](https://leetcode.com/problems/permutations-ii/)

‎0048.rotate-image/README.md

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# [48. Rotate Image](https://leetcode.com/problems/rotate-image/)

‎0048.rotate-image/RotateImage.java

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class RotateImage {
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public void rotate(int[][] matrix) {
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int n = matrix.length;
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for (int i = 0; i < (n + 1) / 2; i++) {
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for (int j = 0; j < n / 2; j++) {
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int temp = matrix[n - 1 - j][i];
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matrix[n - 1 - j][i] = matrix[n - 1 - i][n - j - 1];
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matrix[n - 1 - i][n - j - 1] = matrix[j][n - 1 -i];
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matrix[j][n - 1 -i] = matrix[i][j];
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matrix[i][j] = temp;
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}
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}
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}
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}

‎0049.group-anagrams/GroupAnagrams.java

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import java.util.*;
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class GroupAnagrams {
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public List<List<String>> groupAnagrams(String[] strs) {
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if (strs == null || strs.length == 0) return new ArrayList<>();
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Map<String, List<String>> map = new HashMap<>();
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for (String s : strs) {
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char[] chars = s.toCharArray();
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Arrays.sort(chars);
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String keyString = String.valueOf(chars);
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if (!map.containsKey(keyString)) map.put(keyString, new ArrayList<>());
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map.get(keyString).add(s);
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}
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return new ArrayList<>(map.values());
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}
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}

‎0049.group-anagrams/README.md

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# [49. Group Anagrams](https://leetcode.com/problems/group-anagrams/)

‎0053.maximum-subarray/MaximumSubarray.java

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class MaximumSubarray {
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public int maxSubArray(int[] nums) {
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if (nums == null || nums.length == 0) return 0;
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int preSum = nums[0];
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int maxSum = preSum;
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for (int i = 1; i < nums.length; i++) {
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preSum = preSum > 0 ? preSum + nums[i] : nums[i];
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maxSum = Math.max(preSum, maxSum);
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}
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return maxSum;
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}
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}

‎0053.maximum-subarray/README.md

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# [53. Maximum Subarray](https://leetcode.com/problems/maximum-subarray/)

‎0054.spiral-matrix/README.md

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# [54. Spiral Matrix](https://leetcode.com/problems/spiral-matrix/)

‎0054.spiral-matrix/SpiralMatrix.java

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import java.util.ArrayList;
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import java.util.List;
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class SpiralMatrix {
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public List<Integer> spiralOrder(int[][] matrix) {
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List<Integer> res = new ArrayList<>();
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if (matrix.length == 0) return res;
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int rowBegin = 0;
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int rowEnd = matrix.length - 1;
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int colBegin = 0;
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int colEnd = matrix[0].length - 1;
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while (rowBegin <= rowEnd && colBegin <= colEnd) {
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// right
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for (int j = colBegin; j <= colEnd; j++) {
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res.add(matrix[rowBegin][j]);
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}
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rowBegin++;
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// down
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for (int j = rowBegin; j <= rowEnd; j++) {
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res.add(matrix[j][colEnd]);
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}
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colEnd--;
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// left
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if (rowBegin <= rowEnd) {
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for (int j = colEnd; j >= colBegin; j--) {
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res.add(matrix[rowEnd][j]);
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}
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}
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rowEnd--;
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// top
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if (colBegin <= colEnd) {
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for (int j = rowEnd; j >= rowBegin; j--) {
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res.add(matrix[j][colBegin]);
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}
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}
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colBegin++;
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}
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return res;
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}
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}

‎0055.jump-game/JumpGame.java

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class JumpGame {
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public boolean canJump(int[] nums) {
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int n = nums.length, rightMost = 0;
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for (int i = 0; i < n; i++) {
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if (i <= rightMost) {
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rightMost = Math.max(rightMost, i + nums[i]);
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if (rightMost >= n - 1) return true;
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}
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}
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return false;
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}
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}

‎0055.jump-game/README.md

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# [55. Jump Game](https://leetcode.com/problems/jump-game/)
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Complexity Analysis:
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- Time Complexity: $O(n)$. $n$ is the length of `nums` array.
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- Space Complexity: $O(1)$. Only constant variables.

‎0056.merge-intervals/MergeIntervals.java

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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Comparator;
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import java.util.List;
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class MergeIntervals {
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public int[][] merge(int[][] intervals) {
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if (intervals.length <= 1) return intervals;
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Arrays.sort(intervals, Comparator.comparingInt(a -> a[0]));
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int start = intervals[0][0];
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int end = intervals[0][1];
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List<int[]> res = new ArrayList<>();
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for (int i = 1; i < intervals.length; i++) {
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if (intervals[i][0] > end) {
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res.add(new int[]{start, end});
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start = intervals[i][0];
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}
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end = Math.max(end, intervals[i][1]);
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if (i == intervals.length - 1) {
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res.add(new int[]{start, end});
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}
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}
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return res.toArray(new int[0][]);
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}
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}

‎0056.merge-intervals/README.md

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# [56. Merge Intervals](https://leetcode.com/problems/merge-intervals/)

‎0057.insert-interval/InsertInterval.java

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import java.util.ArrayList;
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import java.util.List;
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class InsertInterval {
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public int[][] insert(int[][] intervals, int[] newInterval) {
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List<int[]> ansList = new ArrayList<>();
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int i = 0;
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// add all the intervals before newInterval starts
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while (i < intervals.length && intervals[i][1] < newInterval[0]) {
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ansList.add(intervals[i]);
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i++;
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}
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// merge intervals with the insert interval
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while (i < intervals.length && intervals[i][0] <= newInterval[1]) {
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newInterval[0] = Math.min(newInterval[0], intervals[i][0]);
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newInterval[1] = Math.max(newInterval[1], intervals[i][1]);
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i++;
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}
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ansList.add(newInterval);
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// add the rest of intervals
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while (i < intervals.length) {
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ansList.add(intervals[i]);
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i++;
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}
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int[][] ans = new int[ansList.size()][2];
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for (int j = 0; j < ansList.size(); j++)
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ans[j] = ansList.get(j);
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return ans;
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}
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}

‎0057.insert-interval/README.md

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# [57. Insert Interval](https://leetcode.com/problems/insert-interval/)
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Complexity Analysis:
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- Time Complexity: $O(n)$. $n$ is the length of intervals.
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- Space Complexity: $O(1)$. We only use extra space for variables.

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