树与图的宽度优先遍历例题

#include <iostream>
#include <algorithm>
#include <cstring>
 
using namespace std;
 
const int N = 100010;
 
int n, m;
int h[N], e[N], ne[N], idx;
int d[N],q[N];
 
 
int ans = N;
 
void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx ++ ;
}
 
int bfs()
{
    int hh = 0, tt = 0;
    q[0] = 1;
 
    memset(d, -1, sizeof d); 
 
    d[1] = 0;
 
    while(hh <= tt)
    {
        int t = q[hh ++];
        
        for(int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            if(d[j] == -1)
            {
                d[j] = d[t] + 1;
                q[ ++ tt] = j; 
            }
        }
    }
    return d[n];
}
 
int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
 
    for(int i = 0; i < m; i ++)
    {
        int a, b;
        cin >> a >> b;
        add(a, b);
    }
 
    cout << bfs() << endl;
 
    return 0;
}