题目描述

输入样例

4 5
1 2
2 3
3 4
1 3
1 4

输出样例

1

参考代码

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 100010;

int n, m;
int h[N], e[N], ne[N], idx;
int d[N], q[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;
}