bfs-求图中点的层次

重点
1. 点的层次和边权没有关系，从源点出发每次向外扩展一层，第一次遇到某个点的时候，这个路径就是点的层次

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std;
const int N = 100010, M = N * 2;
int h[N], ne[M], e[M], cnt;
bool st[N];
int dist[N];
int n, m;

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

void bfs(int u) {
    queue<int> q;
    memset(dist, 0x3f, sizeof(dist));
    q.push(u);
    dist[u] = 0;
    st[u] = true;
    while (q.size()) {
        auto t = q.front();
        q.pop();

        int dis = dist[t];
        for (int i = h[t]; ~i; i = ne[i]) {
            int j = e[i];
            if (st[j]) continue;
            dist[j] = dis + 1;
            st[j] = true;
            q.push(j);
        }
    }
}

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);
    }
    bfs(1);
    if (dist[n] == 0x3f3f3f3f) cout << -1 << endl;
    else cout << dist[n] << endl;
    return 0;
}