【LeetCode】215. Kth Largest Element in an Array (2 solutions)

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.

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

解法一：

先排序，复杂度O(nlogn)

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        sort(nums.begin(), nums.end());
        return nums[nums.size()-k];
    }
};

解法二：

建最大堆，取k次最大元素。复杂度O(n+klogn)

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        make_heap(nums.begin(), nums.end());
        int ret;
        int n = nums.size();
        while(k --)
        {
            pop_heap(nums.begin(), nums.begin()+n);
            ret = nums[n-];
            n --;
        }
        return ret;
    }
};