先有一份代码

#include<bits/stdc++.h>
#define int long long
using namespace std;
int n,m;
basic_string<int>g[111111];
queue<pair<int,int>>q;

int s,t;
// bitset<111111>vis[4];
bool vis[111111][4];
signed main()
{
//	freopen("min_dis.in","r",stdin);
//	freopen("min_dis.out","w",stdout);
	cin>>n>>m;
	for(int i=1,x,y;i<=m;i++)
	{
		cin>>x>>y;
		g[x]+=y;
	}
	cin>>s>>t;
	vis[s][0]=1;
	q.push({s,0});
	while(!q.empty())
	{
		pair<int,int>tt=q.front();
		q.pop();
        tt.second++;
		for(int v:g[tt.first])
		{
			if(!vis[v][tt.second%3])
			{
				if(v==t and tt.second%3==0)
				{
					cout<<tt.second/3;
					return 0;
				}
				vis[v][tt.second%3]=1;
				q.emplace(v,tt.second);
			}
		}
	}
	puts("-1");
}

样例

6 8
1 2
2 3
3 4
4 5
5 1
1 4
1 5
4 6
1 6

用bitset的RE，用bool的AC，为什么