399.cpp

class Solution {
public:
    vector<double> calcEquation(vector<pair<string, string>> equations, vector<double>& values, vector<pair<string, string>> queries) {
        unordered_map<string, unordered_map<string, double>> g;
        for(int i = 0; i < equations.size(); i++) {
            const string& A = equations[i].first;
            const string& B = equations[i].second;
            const double k = values[i];
            g[A][B] = k;
            g[B][A] = 1.0 / k;
        }
        vector<double> ans;
        for(const auto& pair: queries) {
            const string& X = pair.first;
            const string& Y = pair.second;
            if (!g.count(X) || !g.count(Y)) {
                ans.push_back(-1.0);
                continue;
            }
            unordered_set<string> visited;
            ans.push_back(divide(X, Y, g, visited));
        }
        return ans;
    }
    double divide(const string& A, const string& B, unordered_map<string, unordered_map<string, double>>& g,
                 unordered_set<string>& visited) {
        if (A == B) return 1.0;
        visited.insert(A);
        for(const auto& pair: g[A]) {
            const string& C = pair.first;
            if (visited.count(C)) continue;
            double d = divide(C, B, g, visited);
            if (d > 0) return d * g[A][C];
        }
        return -1.0;
    }
};