L2-3 深入虎穴 (25分)

#include<iostream>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<cstring>
#include<string>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;
#define STDIN freopen("in.in", "r", stdin);freopen("out.out", "w", stdout);


const int N = 100000 + 10;
int h[N], e[N*100], ne[N*100], idx;
int du[N];
void add(int a, int b)
{
    e[idx] = b;
    ne[idx] = h[a];
    h[a] = idx++;
}

int ans = 1, de = 0;
void dfs(int u, int d)
{
    if (d > de) {
        ans = u, de = d;
    }
    for (int i = h[u]; ~i; i = ne[i])
    {
        int j = e[i];
        dfs(j, d + 1);
    }
}
int main()
{
    STDIN
    int n; 
    cin >> n;
    memset(h, -1, sizeof h);
    add(0,1);
    for (int i = 1; i <= n; i++)
    {
        int k; scanf("%d", &k);
        int x;
        for (int j = 1; j<= k; j++)
        {
            scanf("%d", &x);
            add(i, x);
            du[x]++;
        }
    }
    int ru;
    for (int i = 1; i <= n; i++) {
        if (!du[i])
        {
            ru = i;
            break;
        }
    }
    dfs(ru, 0);
    cout << ans << endl;
}