QuickSort.java

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 */

// Merge Sort

public class Solution {

  public void quickSort(int[] a) {
    if (a == null || a.length == 0) { return; }

    sort(a, 0, a.length - 1);
  }

  private void sort(int[] a, int start, int end) {
    if (start >= end) { return; }

    // two pointers
    int left = start, right = end;
    int pivot = a[(start + end) / 2];

    // partition
    // pivot could be in both left and right partition
    while (left <= right) {
      // find first position that left side violate a[left] < pivot
      while (left <= right && a[left] < pivot) { ++left; }
      // find first position that right side violate a[right] > pivot
      while (left <= right && a[right] > pivot) { --right; }
      // swap
      if (left <= right) {
        int temp = a[left];
        a[left] = a[right];
        a[right] = temp;
        left++;
        right--;
      }
    }

    // right now, left is right, and right is left
    sort(a, start, right);
    sort(a, left, end);
  }
}