373. Find K Pairs with Smallest Sums

You are given two integer arrays nums1 and nums2 sorted in ascending order and an integer k.

Define a pair (u,v) which consists of one element from the first array and one element from the second array.

Find the k pairs (u1,v1),(u2,v2) ...(uk,vk) with the smallest sums.

Example 1: Given nums1 = [1,7,11], nums2 = [2,4,6], k = 3

Return: [1,2],[1,4],[1,6]

The first 3 pairs are returned from the sequence:
[1,2],[1,4],[1,6],[7,2],[7,4],[11,2],[7,6],[11,4],[11,6]

Example 2: Given nums1 = [1,1,2], nums2 = [1,2,3], k = 2

Return: [1,1],[1,1]

The first 2 pairs are returned from the sequence:
[1,1],[1,1],[1,2],[2,1],[1,2],[2,2],[1,3],[1,3],[2,3]

Example 3: Given nums1 = [1,2], nums2 = [3], k = 3

Return: [1,3],[2,3]

All possible pairs are returned from the sequence:
[1,3],[2,3]

定义最小堆，此处要实现比较函数

class Solution {
    typedef pair<int, int> P;
    struct cmp{
        bool operator()(P a, P b){
            return a.first + a.second > b.first + b.second;
        }
    };
public:
    vector<pair<int, int>> kSmallestPairs(vector<int>& nums1, vector<int>& nums2, int k) {
        if(nums1.empty() || nums2.empty())  return {};
        priority_queue<P, vector<P>, cmp> min_heap;
        for(auto &i:nums1)
            for(auto &j:nums2){
                min_heap.push(make_pair(i, j));
            }

        vector<P> res;    
        while(k-->0 && min_heap.size() > 0) { 
                res.push_back(min_heap.top());
                min_heap.pop();

        }
        return res;
    }
};