Kth Largest Element in an Array

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本文介绍了一种高效的方法来找到未排序数组中第K大的元素，通过使用最小堆或快速排序的变形来实现。这种方法适用于各种场景，包括大数据集的处理。

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);
    }
};