Evaluate Division

Equations are given in the format A / B = k, where A and B are variables represented as strings, and k is a real number (floating point number). Given some queries, return the answers. If the answer does not exist, return -1.0.

Example: Given a / b = 2.0, b / c = 3.0. queries are: a / c = ?, b / a = ?, a / e = ?, a / a = ?, x / x = ? . return [6.0, 0.5, -1.0, 1.0, -1.0 ].

The input is: vector<pair<string, string>> equations, vector<double>& values, vector<pair<string, string>> queries , where equations.size() == values.size(), and the values are positive. This represents the equations. Return vector<double>.

According to the example above:

equations = [ ["a", "b"], ["b", "c"] ],
values = [2.0, 3.0],
queries = [ ["a", "c"], ["b", "a"], ["a", "e"], ["a", "a"], ["x", "x"] ]. 

The input is always valid. You may assume that evaluating the queries will result in no division by zero and there is no contradiction.

分析

建图，分别{积：divisor1,devisor2} 2个都加入

dfs从起点起一步步拆分start，同时*product.注意这里product起始是1.0不是1

class Solution:
    def calcEquation(self, equations: List[List[str]], values: List[float], queries: List[List[str]]) -> List[float]:
        G = collections.defaultdict(list)
        for e,v in zip(equations, values):
            a,b = e
            G[a].append((b,v))
            G[b].append((a,1/v))            
        
        def dfs(s,e,p,visited):
            if s not in G or e not in G or s in visited:
                return -1
            if s == e:
                return p
            visited.add(s)
            for n,v in G[s]:
                temp = dfs(n,e,p*v,visited)
                if temp!=-1:
                    return temp
            return -1
 
        return[dfs(a,b,1.0,set()) for a,b in queries]