1010.NumPairsDivisibleBy60.cs

// 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.

// Example 1:
// Input: time = [30,20,150,100,40]
// Output: 3
// Explanation: Three pairs have a total duration divisible by 60:
// (time[0] = 30, time[2] = 150): total duration 180
// (time[1] = 20, time[3] = 100): total duration 120
// (time[1] = 20, time[4] = 40): total duration 60

// Example 2:
// Input: time = [60,60,60]
// Output: 3
// Explanation: All three pairs have a total duration of 120, which is divisible by 60.

// Constraints:
// 1 <= time.length <= 6 * 104
// 1 <= time[i] <= 500

public class Solution {
    public int NumPairsDivisibleBy60(int[] time) {
        Dictionary<int, int> modTime = new();
        int res = 0;

        foreach (var t in time) {
            int mod = t % 60;
            int complement = (mod == 0) ? 0 : 60 - mod;

            // Add the count of previous complements
            if (modTime.ContainsKey(complement))
                res += modTime[complement];

            // Increment count for the current modulus
            if (!modTime.ContainsKey(mod))
                modTime[mod] = 0;

            modTime[mod]++;
        }

        return res;
    }
}