215. Kth Largest Element in an Array

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note:
You may assume k is always valid, 1 ? k ? array's length.

一刷
Quick select
最好的情况下：
Discard half each time: n+(n/2)+(n/4)..1 = n + (n-1) = O(2n-1) = O(n), because n/2+n/4+n/8+..1=n-1.
原理是每次随机取一个pivot, 然后把大于pivot的都放在右边，小于pivot的都放在左边。如果pivot就在k的位置，直接返回，否则缩小范围继续上面的步骤

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        if(nums == null || nums.length == 0) return Integer.MAX_VALUE;
        //nums.length - k, the start position of last k
        return find(nums, 0, nums.length-1, nums.length - k);
    }
    
    private int find(int[] nums, int start, int end, int k){
        if(start>end) return Integer.MAX_VALUE;
        int pivot = nums[end];
        int left = start;
        for(int i=start; i