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.

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