If we capture a map by value, then we can’t use the operator []:
unordered_map<int, int> freq;
// Won't compile
// auto comp_by_map = [freq](const int& a, const int& b) { return freq[a] < freq[b];};
auto comp_by_map = [&freq](const int& a, const int& b) { return freq[a] < freq[b];};
tags: Hash Table,Heap (data structure),LeetCode101,Priority Queue,C++ Lambda class Solution { public: vector<int> topKFrequent(vector<int>& nums, int k) { vector<int> res; unordered_map<int, int> freq; // Note that: we need caputre a map by reference, // otherwise we can't use the operator[]. // See also: https://stackoverflow.com/a/6281071 auto comp_by_map = [&freq](const int& a, const int& b) { return freq[a] < freq[b]; }; // Note that: here we need pass our lambda /comp_by_map/ to the // constructor of std::priority_queue....