3780. Maximum Sum of Three Numbers Divisible by Three

You are given an integer array nums.

Your task is to choose exactly three integers from nums such that their sum is divisible by three.

Return the maximum possible sum of such a triplet. If no such triplet exists, return 0.

class Solution {
public:
    int maximumSum(vector<int>& nums) {
        unordered_map<int,vector<int>> S;
        for(auto& n : nums) S[n % 3].push_back(n);
        for(auto& [k,v] : S) sort(begin(v), end(v));
        int res = 0;
        for(int i = 0; i < 3; i++) {
            if(S[i].size() >= 3) {
                int sz = S[i].size();
                res = max(res, S[i][sz-1] + S[i][sz-2] + S[i][sz-3]);
            }
        }
        
        if(S[0].size() and S[1].size() and S[2].size()) {
            res = max(res, S[0].back() + S[1].back() + S[2].back());
        }
        return res;
    }
};