quick select 及其相关问题

 1 public class Solution {
 2     public int findKthLargest(int[] nums, int k) {
 3         if (nums == null || nums.length == 0) {
 4             return -1;
 5         }
 6         return helper(nums, nums.length - k + 1, 0, nums.length - 1);  //length - k + 1 convert the kth large to k‘th small one      
 7     }
 8     private int helper(int[] nums, int k, int start, int end) {
 9         if (start == end) {
10             return nums[start];
11         }
12         int position = partition(nums, start, end);
13         if (position + 1 == k) {
14             return nums[position];
15         }else if (position + 1 > k) {
16             return helper(nums, k, start, position - 1); //这个地方为什么是return 下一层递归的结果，因为需要的结果在下层／下下层递归中得到，
　　　　　　　　　　把这个值返回来交给最上面的一层
17         } else {
18             return helper(nums, k, position + 1, end);
19         }
20     }
21     
22     private int partition(int[] nums, int left, int right) {
23         int pivot = nums[left];
24         while (left < right) {
25              while (left < right && nums[right] >= pivot) {
26                 right--;
27             }
28             nums[left] = nums[right];
29             while (left < right && nums[left] <= pivot) {
30                 left++;
31             }
32             nums[right] = nums[left];
33         }
34         nums[left] = pivot;
35         return left;
36     }
37 }