【BFS】AcWing847.图中点的层次

AcWing847.图中点的层次

题解

#include <iostream>
#include <cstring>
#include <queue>
#include <cstdio>

using namespace std;

const int N = 1e5 + 10;

int n, m;
queue<int> q;
int h[N], ne[2 * N], e[2 * N], idx, d[N];

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

void BFS()
{
    q.push(1);
    d[1] = 0;
    while(q.size())
    {
        int a = q.front();
        if(a == n) break;
        q.pop();
        for(int i = h[a]; ~i; i = ne[i])
            if(d[e[i]] == -1)
                q.push(e[i]), d[e[i]] = d[a] + 1;
    }
}

int main()
{
    memset(h, -1, sizeof h);
    memset(d, -1, sizeof d);
    int n, m, a, b;
    scanf("%d%d",&n,&m);
    for(int i = 0; i < m; ++i)
    {
        scanf("%d%d",&a, &b);
        add(a, b);
    }
    BFS();
    cout << d[n] << endl;
    return 0;
}