1. 根据 connections 建立无向树
2. 从 0 开始深搜，每次调用 dfs 时判断路径方向是否正确

class Solution:
    def minReorder(self, n: int, connections: List[List[int]]) -> int:
        to = defaultdict(set)
        edge = defaultdict(list)
        for con in connections:
            edge[con[0]].append(con[1])
            edge[con[1]].append(con[0])
            to[con[0]].add(con[1])
        
        ans = 0
        def dfs(node, fa):
            nonlocal ans
            for ch in edge[node]:
                if ch == fa:
                    continue
                if ch in to[node]:
                    ans += 1
                dfs(ch, node)
        
        dfs(0, -1)
        return ans