hdu 1548 A strange lift

http://acm.hdu.edu.cn/showproblem.php?pid=1548

#include <cstdio>
#include <iostream>
#include <queue>
#include <cstring>
#include <algorithm>
#define maxn 300
using namespace std;

int g[maxn][maxn];
int n;
int A,B;
bool vis[maxn];
int step[maxn];
int a[maxn];
bool flag=true;

void bfs(int src)
{
    memset(vis,false,sizeof(vis));
    queue<int>q;
    q.push(src);
    step[src]=0;
    vis[src]=true;
    while(!q.empty())
    {
        int u=q.front(); q.pop();
        if(u==B)
        {
            flag=false;
            return ;
        }
        int next=u+a[u];
        if(next>=1&&next<=n&&!vis[next])
        {
            step[next]=step[u]+1;
            vis[next]=true;
            q.push(next);
        }
        next=u-a[u];
        if(next>=1&&next<=n&&!vis[next])
        {
            step[next]=step[u]+1;
            vis[next]=true;
            q.push(next);
        }
    }
}
int main()
{
    while(scanf("%d",&n)!=EOF)
    {
        if(n==0) break;
        scanf("%d%d",&A,&B);
        for(int i=1; i<=n; i++)
        {
            cin>>a[i];
        }
        flag=true;
        bfs(A);
        if(!flag)
        printf("%d
",step[B]);
        else
        printf("-1
");
    }
    return 0;
}