有向图（广搜）

#include<stdio.h>

struct note
{
	int x;
	int s;
};

int mian()
{
	struct note que[2501];
	int e[51][51]={0},book[51]={0};
	int head,tail;
	int i,j,m,n,a,b,cur,start,end,flag=0;
	scanf("%d %d %d %d",&n,&m,&start,&end);
	for(i=1;i<=n;i++)
	     for(j=1;j<=n;j++)
	        if(i==j) e[i][j]=0;
	        else e[i][j]=99999999;
	for(i=1;i<=m;i++)
	{
		scanf("%d %d",&a,&b);
		e[a][b]=1;
		e[b][a]=1;
	}
	head=1;
	head=1;
	
	que[tail].x=start;
	que[tail].s=0;
	tail++;
	book[1]=start;
	while(head<tail)
	{
		cur=que[head].x;
		for(j=1;j<=n;j++)
		{
			if(e[cur][j]=99999999&&book[j]==0)
			{
				que[tail].x=j;
				que[tail].s=que[head].s+1;
				tail++;
				book[j]=1;
			}
	if(que[tail].x==end)
	{
		flag=1;
		break;
	}
	if(flag==1)
	break;
	head++;
}
    printf("%d",que[tail-1].s);
    return 0;
	}
	    
}