LeetCode 378. Kth Smallest Element in a Sorted Matrix

class Solution {public:    int kthSmallest(vector<vector<int>>& matrix, int k) {        vector<int> a;        vector<int>::iterator is;        vector<vector<int>>::iterator it;        for(it=matrix.begin();it!=matrix.end();it++)        {            for(is=(*it).begin();is!=(*it).end();is++)            {                a.push_back(*is);            }        }        sort(a.begin(),a.end());        return a[k-1];    }};