"Kth Largest Element in an Array": another method

Author: lennylxx

src/215.c

@@ -34,6 +34,46 @@ int findKthLargest(int* nums, int numsSize, int k) {
     }
 }
 
+void swap(int *a, int *b) {
+    int t = *a;
+    *a = *b;
+    *b = t;
+}
+
+/* another method, diff from Hoare's quicksort */
+int findKthLargest0(int* nums, int numsSize, int k) {
+    if (k < 1 || k > numsSize) return 0;
+
+    int pivotIndex = 0; /* select a pivot, simply use leftest elem */
+    int pivotValue = nums[pivotIndex];
+    int i = 0; /* store index */
+    int j; /* sweep index */
+
+    swap(&nums[pivotIndex], &nums[numsSize - 1]);
+
+    for (j = 0; j < numsSize - 1; j++) {
+        if (nums[j] <= pivotValue) {
+            swap(&nums[j], &nums[i]);
+            i++;
+        }
+    }
+
+    swap(&nums[i], &nums[numsSize - 1]);
+
+    int rightSize = numsSize - i - 1; /* size of right sub array */
+
+    if (rightSize + 1 == k) { /* found, it's pivot */
+        return nums[i];
+    }
+
+    if (rightSize >= k) {
+        return findKthLargest0(nums + i + 1, rightSize, k); /* find in right half */
+    }
+    else {
+        return findKthLargest0(nums, i, k - rightSize - 1); /* find in left half */
+    }
+}
+
 int main() {
 
     int n[] = { 3, 7, 8, 1, 2, 5, 6, 9 };