[LeetCode] Kth Largest Element in an Array

Find the kth largest element in an unsorted array.

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.

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

利用快排的思想，平均时间复杂度O(n)，4ms AC。最坏情况下会退化成O(n^2)，比如说数组是从小到大排好序的而要找的是最大值。非递归，空间复杂度O(1)。

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        int L = 0, R = nums.size() - 1;
        while (L < R) {
            int left = L, right = R;
            int key = nums[left];
            while (left < right) {
                while (left < right && nums[right] < key) --right;
                nums[left] = nums[right];
                while (left < right && nums[left] >= key) ++left;
                nums[right] = nums[left];
            }
            nums[left] = key;
            if (left == k - 1) return nums[k - 1];
            else if (left > k - 1) R = left - 1;
            else L = left + 1;
        }
        return nums[k - 1];
    }
};