跪求大佬辅导

这道题与P2279，P3942不应该是同类型的题吗？为什么自底向上贪心会错呢？（看到好几个人都是用P2279第一篇题解的方法WA四个点）另附代码

#include <bits/stdc++.h>

using namespace std;

#define N 2010

int n, cnt, ans;
int head[N], to[N << 1], nxt[N << 1];
int depth[N], fa[N], dfn[N], dis[N];

bool compare(int x, int y) {
    return depth[x] > depth[y]; 
}

inline void add(int x, int y) {
    to[++cnt] = y, nxt[cnt] = head[x], head[x] = cnt;   
}

void dfs(int now, int f) {
    fa[now] = f, depth[now] = depth[f] + 1;
    for (int i = head[now]; i; i = nxt[i]) {
        if (to[i] == f) continue;
        dfs(to[i], now);    
    }
}

int main() {

//  freopen("in.in", "r", stdin);

    scanf("%d", &n);

    int u, k, x;
    for (int i = 1; i <= n; ++i) {
        scanf("%d%d", &u, &k);
        for (int j = 1; j <= k; ++j) {
            scanf("%d", &x);
            add(u + 1, x + 1), add(x + 1, u + 1);   
        }
    }

    dfs(1, 0); 

    for (int i = 1; i <= n; ++i)    dfn[i] = i;

    memset(dis, 0x3f, sizeof dis);
    sort(dfn + 1, dfn + n + 1, compare);

    for (int i = 1; i <= n; ++i) {
        int now = dfn[i], f = fa[now];
        dis[now] = min(dis[now], dis[f] + 1);
        if (dis[now] > 1) {
            dis[f] = 0, dis[fa[f]] = min(dis[fa[f]], 1);
            ++ans;
        }
    }

    printf("%d\n", ans);

    return 0;
}