树上逃离-最短路

#include <bits/stdc++.h>

using namespace std;

const int N = 100010;

int n, m, k, p;
int h[N], e[N], ne[N], idx;
int b[N];
string path[N], res[N];

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

void bfs()
{
    queue<int> q;
    q.push(0);
    path[0] = "0";
    while(q.size())
    {
        int t = q.front();
        q.pop();
        int idx = 0;
        for(int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            if(!b[j])
            {
                q.push(j);
                path[j] = path[t] + "->" + to_string(j);
                if(h[j] == -1)
                {
                    idx = 1;
                    res[p ++] = path[j];
                }
            }
        }
        if(idx)
        {
            sort(res, res + p);
            cout << res[0] << endl;
            return;
        }
    }
    cout << "NULL" << endl;
}

int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
    while(m --)
    {
        int a, b;
        cin >> a >> b;
        add(a, b);
    }
    cin >> k;
    while(k --)
    {
        int x;
        cin >> x;
        b[x] = 1;
    }
    bfs();
    return 0;
}