LeetCode 215 Kth Largest Element in an Array

思路： 跟找第kth小的数相反，数组逆向排序。从前找第k － 1个数。

 

public class Solution {
    public int findKthLargest(int[] array, int k) {
        // Write your solution here
        if (array == null || array.length == 0) {
          return -1;
        }

        quickSelect(array, 0, array.length - 1, k);
        return array[k - 1];
      }

      private void quickSelect(int[] array, int start, int end, int k) {
        if (start >= end) {
          return;
        }
        Random rand = new Random();
        int pivot = start + rand.nextInt(end - start + 1);
        swap(array, pivot, start);

        int left = start + 1, right = end;
        while (left <= right) {
          if (array[right] < array[start]) {
            right--;
          } else {
            swap(array, left, right);
            left++;
          }
        }
        //最后left跟right错开后，left仍然指着第一个小于pivot的元素，right指着最后一个大于pivot的元素
        swap(array, right, start);
        if (k == right) {
          return;
        } else if (k < right) {
          quickSelect(array, start, right - 1, k);
        } else if (k > right) {
          quickSelect(array, right + 1, end, k);
        }
      }

      private void swap(int[] array, int start, int end) {
        int temp = array[start];
        array[start] = array[end];
        array[end] = temp;
      }
}