LeetCode #215 - Kth Largest Element in an Array

题目描述：

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.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5

Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4

Note: 
You may assume k is always valid, 1 ≤ k ≤ array's length.

由于是无序数组，直接排序再求解的时间复杂度为O(nlog n)，但是其实我们只需要求第k大的元素即可，所以我们只需要维持一个程度为k的优先队列，遍历一遍数组并进行插入和删除，时间复杂度为O(nlog k)。

class comp{
    public:
    bool operator()(int a,int b)
    {
        if(a>b) return true;
        else return false;
    }
};

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        priority_queue<int,vector<int>,comp> q;
        for(int i=0;i<nums.size();i++)
        {
            if(q.size()<k) q.push(nums[i]);
            else
            {
                q.push(nums[i]);
                q.pop();
            }
        }
        return q.top();
    }
};