Leetcode-215. Kth Largest Element in an Array

地址：https://leetcode.com/problems/kth-largest-element-in-an-array/

描述：

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.

实例1：

Input: [3,2,1,5,6,4] and k = 2
Output: 5

实例2：

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4

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

方法：

public int findKthLargest(int[] nums, int k) {
        int start = 0, end = nums.length - 1, index = nums.length - k;
        while (start < end) {
            int pivot = partion(nums, start, end);
            if (pivot < index) start = pivot + 1;
            else if (pivot > index) end = pivot - 1;
            else return nums[pivot];
        }
        return nums[start];
    }

    private int partion(int[] nums, int start, int end) {
        int pivot = start, temp;
        while (start <= end) {
            while (start <= end && nums[start] <= nums[pivot]) start++;
            while (start <= end && nums[end] > nums[pivot]) end--;
            if (start > end) break;
            temp = nums[start];
            nums[start] = nums[end];
            nums[end] = temp;
        }
        temp = nums[end];
        nums[end] = nums[pivot];
        nums[pivot] = temp;
        return end;
    }