1010. Pairs of Songs With Total Durations Divisible by 60

You are given a list of songs where the ith song has a duration of time[i] seconds.

Return the number of pairs of songs for which their total duration in seconds is divisible by 60. Formally, we want the number of indices i, j such that i < j with (time[i] + time[j]) % 60 == 0.

class Solution {
public:
    int numPairsDivisibleBy60(vector<int>& time) {
        unordered_map<int, int> m;
        for(auto& t : time) {
            m[t % 60]++;
        }

        int res = m[0] * (m[0] - 1) / 2 + m[30] * (m[30] - 1) / 2;
        
        for(int i = 1; i <= 29; i++) {
            res += m[i] * m[60 - i];
        }

        return res;
    }
};