宽度优先遍历（模板）

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;
}