### minReorder 
from collections import defaultdict
from typing import *
def f_gold(n: int, connections: List[List[int]]) -> int:
    def dfs(u):
        vis[u] = True
        ans = 0
        for v in g[u]:
            if not vis[v]:
                if (u, v) in s:
                    ans += 1
                ans += dfs(v)
        return ans
    g = defaultdict(list)
    s = set()
    for a, b in connections:
        g[a].append(b)
        g[b].append(a)
        s.add((a, b))
    vis = [False] * n
    return dfs(0)