P1135 奇怪的电梯

spfa

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 #define maxn 1000000
 4 #define inf 1<<30
 5 int n,m,x,y,z,num,head[maxn],dis[maxn],a,b;
 6 struct Edge{
 7     int u,v,d,next;
 8 }edge[maxn];
 9 char ch;
10 inline void read(int &now)
11 {
12     ch=getchar(); now=0;
13     while(ch>‘9‘||ch<‘0‘) ch=getchar();
14     while(ch>=‘0‘&&ch<=‘9‘) now=now*10+ch-‘0‘,ch=getchar();
15 }
16 void add_Edge(int u,int v,int d)
17 {
18     edge[++num].v=v;
19     edge[num].d=d;
20     edge[num].next=head[u];
21     head[u]=num;
22 }
23 void spfa(int now)
24 {
25     queue<int>que;
26     bool vis[maxn];
27     for(int i=1;i<=n;i++) dis[i]=inf,vis[i]=false;
28     dis[now]=0,vis[now]=true;
29     que.push(now);
30     while(!que.empty())
31     {
32         int cur=que.front(); que.pop();
33         for(int i=head[cur];i;i=edge[i].next)
34         {
35             if(dis[edge[i].v]>dis[cur]+edge[i].d)
36             {
37                 dis[edge[i].v]=dis[cur]+edge[i].d;
38                 if(!vis[edge[i].v])
39                 {
40                     vis[edge[i].v]=true;
41                     que.push(edge[i].v);
42                 }
43             }
44         }
45         vis[cur]=false;
46     }
47 }
48 int main()
49 {
50     read(n); read(a); read(b);
51     for(int i=1;i<=n;i++)
52     {
53         read(x); if(i-x>0) add_Edge(i,i-x,1);
54         if(i+x<=n) add_Edge(i,i+x,1);
55     }
56     spfa(a);
57     if(dis[b]==inf) printf("-1\n");
58     else printf("%d\n",dis[b]);
59     return 0;
60 }