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Author: xuebu

magical&Double pointer/middle-of-the-linked-list-876.java

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+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ * 876. 链表的中间结点 快慢指针法
+ */
+class Solution {
+    public ListNode middleNode(ListNode head) {
+        if (head == null ) return head;
+        ListNode slow = head,fast = head;
+        while(fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        return slow;
+    }
+}

math/monotone-increasing-digits-738.java

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+/**
+ * 先拿到第一个断层，然后从断层开始往回减-1直到后面都不大于前面，再对当前的i位后的所有值填充9
+ */
+class Solution {
+    public int monotoneIncreasingDigits(int N) {
+        char[] res = String.valueOf(N).toCharArray();
+        int i = 1;
+        while (i < res.length && res[i-1] <= res[i]) i++;
+        while (0 < i && i < res.length && res[i-1] > res[i]) res[--i]--;
+        for (int j = i+1; j < res.length; ++j) res[j] = '9';
+
+        return Integer.parseInt(String.valueOf(res));
+    }
+}

string/partition-labels-763.java

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+/**
+ * 贪心算法
+ */
+class Solution {
+    public List<Integer> partitionLabels(String S) {
+        int[] last = new int[26];
+        for(int i = 0 ; i < S.length() ; i++) {
+            last[S.charAt(i) - 'a'] = i;
+        }
+        List<Integer> res = new ArrayList<>();
+        int pos = 0 , anchor = 0;
+        for(int j = 0 ; j < S.length() ; j++) {
+            pos = Math.max(pos,last[S.charAt(j) - 'a']);
+            if (j == pos) {
+                res.add(j - anchor + 1);
+                anchor = j + 1;
+            }
+        }
+        return res;
+    }
+}