除法求值

class Solution:    def calcEquation(self, equations, values, queries):        g = {}                       # 邻接表 g[节点]=[(邻居, 比值)]        for (a, b), v in zip(equations, values):            g.setdefault(a, []).append((b, v))      # a→b 权 v            g.setdefault(b, []).append((a, 1 / v))  # b→a 权 1/v        def dfs(x, y, seen):            if x not in g or y not in g:  # 变量没出现过                return -1.0            if x == y:                    # 走到目标                return 1.0            seen.add(x)            for nei, w in g[x]:           # 看每个邻居                if nei not in seen:                    sub = dfs(nei, y, seen)                    if sub != -1.0:       # 邻居那条路通                        return w * sub    # 累乘比值            return -1.0                   # 所有邻居都不通        return [dfs(a, b, set()) for a, b in queries]

# 1) 一条带权关系 → 正反两条边for (a, b), v in zip(relations, values):    g.setdefault(a, []).append((b, v))      # 正向    g.setdefault(b, []).append((a, 1 / v))  # 反向取倒数# 2) DFS 沿路累乘，三个出口def dfs(x, y, seen):    if x not in g or y not in g: return -1.0  # 出口①不存在    if x == y: return 1.0                     # 出口②到目标    seen.add(x)    for nei, w in g[x]:        if nei in seen: continue              # 防环        sub = dfs(nei, y, seen)        if sub != -1.0: return w * sub        # 累乘并上抛    return -1.0                               # 出口③走不通