703. Kth Largest Element in a Stream*

​​https://leetcode.com/problems/kth-largest-element-in-a-stream/​​

题目描述

Design a class to find the kth largest element in a stream. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Your ​​KthLargest​​​ class will have a constructor which accepts an integer ​​k​​​ and an integer array ​​nums​​​, which contains initial elements from the stream. For each call to the method ​​KthLargest.add​​, return the element representing the kth largest element in the stream.

Example:

int k = 3;
int[] arr = [4,5,8,2];
KthLargest kthLargest = new KthLargest(3, arr);
kthLargest.add(3);   // returns 4
kthLargest.add(5);   // returns 5
kthLargest.add(10);  // returns 5
kthLargest.add(9);   // returns 8
kthLargest.add(4);   // returns 8

Note:
You may assume that ​​​nums​​​’ length ≥ ​​k-1​​​ and ​​k ≥ 1​​.

C++ 实现 1

使用最小堆, 用来保存最大的 k 个数, 其中堆顶为第 k 个最大的数.

class KthLargest {
public:
    int size = 0;
    // 使用最小堆, 保证堆顶为第 k 个最大的数.
    priority_queue<int, vector<int>, std::greater<int>> q;
    KthLargest(int k, vector<int>& nums) {
        for (int i = 0; i < nums.size(); ++ i) {
            q.push(nums[i]);
            if (q.size() > k) q.pop();
        }
        size = k;
    }
    
    int add(int val) {
        q.push(val);
        if (q.size() > size) q.pop();
        return q.top();
    }
};