7-10 病毒溯源 (25 分)

 给定一棵树，找出从根节点到叶节点的最长路径（如果有多个最长路径，输出节点数字小的那个）；

#include 
#include 
using namespace std;
const int N = 10010;
int h[N], e[N], ne[N], idx;
int son[N];
bool st[N];
int n;

void add(int a, int b) {
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

int dfs(int root) {
    int ans = 0;
    son[root] = -1;
    for(int i = h[root]; ~i; i = ne[i]) {
        int j = e[i];
        int d = dfs(j);
        if(ans < d) {
            ans = d;
            son[root] = j;
        } else if(ans == d) {
            son[root] = min(son[root], j);
        }
    }
    return ans + 1;
}
int main() {
    cin >> n;
    memset(h, -1, sizeof h);
    for(int i = 0; i < n; i++) {
        int cnt;
        cin >> cnt;
        while(cnt--) {
            int x;
            cin >> x;
            add(i, x);
            st[x] = true;
        }
    }
    int root = 0;
    while(st[root]) root ++;
    cout << dfs(root) << endl;
    cout << root;
    while(son[root] != -1) {
        cout << ' ' << son[root];
        root = son[root];
    }
    return 0;
}