hdu 1548 A strange lift

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

本题为简单最短路，构图方法为如果在第i层可以上升Ki层则增加边i - >(i+Ki) 否则不加边，如果可以下降Ki层则增加边i-> (i - Ki)

#include<iostream>
#include<queue>
#include<string>
using namespace std;
#define maxn 205
#define INF (1<<30)
struct node
{
	int v;
	node *next;
}*head[maxn],edge[maxn*maxn];
int n,dis[maxn];
bool vis[maxn];
void spfa(int start)
{
	for(int i = 0; i <= n; i++)
		dis[i] = INF;
	memset(vis,false,sizeof(vis));
	vis[start] = true;
	dis[start] = 0;
	queue<int>que;
	que.push(start);
	while(!que.empty())
	{
		int now = que.front();
		vis[now] = true;
		que.pop();
		for(node *p = head[now]; p; p = p->next)
		{
			if(dis[p->v] > dis[now] + 1)
			{
				dis[p->v] = dis[now] + 1;
				if( !vis[p->v])
				{
					vis[p->v] = true;
					que.push(p->v);
				}
			}
		}
	}
}
int main()
{
	int s,e,i,K;
	while(cin >> n,n)
	{
		cin >> s >> e;
		for( i = 0; i <= n; i++)
			head[i] = NULL;
		node *p = edge;
		for(i = 1; i <= n; i++)
		{
			scanf("%d",&K);
			if(i - end >= 1)//如果可以下降K层则增加边i -> i - K
			{
				p->v = i - K;
				p->next = head[i];
				head[i] = p++;
			}
			if(i + end <= n) //如果可以上升K层则增加边i -> i+K
			{
				p->v = i + K;
				p->next = head[i];
				head[i] = p++;
			}
		}
		spfa(s);
		if(dis[e] == INF)
			printf("-1\n");
		else
			printf("%d\n",dis[e]);
	}
	return 0;
}