hdu 1548 A strange lift 最短路（spfa）

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<queue>

using namespace std;

#define N 205
#define maxx 99999999
int a

;
int dis
;
int vis
;
int st, ed;
int n;
int c
;

void spfa()
{
	memset(vis, 0, sizeof(vis));
	queue<int> q;
	while( !q.empty() )
		q.pop();
	dis[st] = 0;
	vis[st] = 1;
	q.push(st);
	while( !q.empty() )
	{
		int now = q.front();
		q.pop();
		vis[now] = 0;
		for( int i = 1; i <= n; i++)
		{
			if( dis[now] + a[now][i] < dis[i] )
			{
				dis[i] = dis[now] + a[now][i];
				if( !vis[i] )
				{
					vis[i] = 1;
					q.push(i);
				}
			}
		}
	}
	if(dis[ed] != maxx)
		printf("%d\n", dis[ed]);
	else
		puts("-1");
}

int main()
{
	while(~scanf("%d", &n) && n)
	{
		scanf("%d%d", &st, &ed);
		for( int i = 1; i <= n; i++ )
		{
			dis[i] = maxx;
			for( int j = 1; j <= n; j++ )
				a[i][j] = maxx;
		}
		for( int i = 1; i <= n; i++)
		{
			scanf("%d", &c[i]);
			if( i + c[i] <= n )
			{
				a[i][c[i] + i] = 1;
			}
			if( i - c[i] >= 1 )
			{
				a[i][i - c[i]] = 1;
			}
		}
		spfa();
	}
	return 0;
}