题目acwing
#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
const int N = 100010;
int n, m;
int h[N], e[N], ne[N], idx;
int q[N], d[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;//1.压入起点
//-1表示未遍历过
memset(d, -1, sizeof d);
d[1] = 0;
while (hh <= tt) {//2.只要队列不空,取出队头
int t = q[hh++];
for (int i = h[t]; i != -1; i = ne[i]) {//3.遍历当前点(队头)的所有邻接点;
int j = e[i];
//4.遍历完重新循环,再取出队头,遍历这个队头的所有邻接点,这样就能从上到下一排一排的插入队列
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;
}