399. 除法求值

class Solution 
{
public:
    vector<double> calcEquation(vector<vector<string>> &equations, vector<double> &values, vector<vector<string>> &queries) 
    {
        unordered_map<string, unordered_map<string, double >> weight;
        for (int i = 0; i < equations.size(); i++) 
        {
            weight[equations[i][0]][equations[i][1]] = values[i];
            weight[equations[i][1]][equations[i][0]] = 1. / values[i];
        }
        vector<double> res;
        for (auto query:queries) 
        {
            res.push_back(bfs(query[0], query[1], weight));
        }
        return res;
    }

    double bfs(string start, string end, unordered_map<string, unordered_map<string, double >> weight) 
    {
        if (weight.count(start) == 0 || weight.count(end) == 0)
            return -1;
        queue<pair<string, double >> que;
        unordered_set<string> visited;
        visited.insert(start);
        que.push(make_pair(start, 1.0));
        while (!que.empty()) 
        {
            int size = que.size();
            for (int i = 0; i < size; i++) 
            {
                auto node = que.front();
                que.pop();
                if (node.first == end)
                    return node.second;
                for (auto mid:weight[node.first]) 
                {
                    if (visited.count(mid.first) == 0) 
                    {
                        visited.insert(mid.first);
                        que.push(make_pair(mid.first, node.second * mid.second));//没找到就一层层往下乘
                    }
                }
            }
        }
        return -1;
    }
};