Kth Largest Element in an Array

Problem

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.

Solution

1. 维护一个大小为K的最小堆，每次和堆的最小值比较

2. 用quick sort 的变形

class Solution {
    int findHelper( vector<int>& nums, int k, int left, int right){
        if(left > right) return INT_MIN;
        int leftCopy = left, rightCopy = right;
        int pivot = nums[left++];
        
        while(left <= right){
            if(nums[left] > pivot){
                left++; continue;
            }
            if(nums[right] <= pivot){
                right--; continue;
            }
            swap(nums[left++], nums[right--]);
        }
        swap(nums[leftCopy], nums[right]);
        
        if(right == k-1) return nums[right];
        if(right < k-1) return findHelper(nums, k, right+1, rightCopy);
        else return findHelper(nums, k, leftCopy, right-1);
    }
public:
    int findKthLargest(vector<int>& nums, int k) {
        return findHelper(nums, k, 0, nums.size()-1);
    }
};