hdu 1548 A strange lift

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

 1 #include <cstdio>

 2 #include <iostream>

 3 #include <queue>

 4 #include <cstring>

 5 #include <algorithm>

 6 #define maxn 300

 7 using namespace std;

 8 

 9 int g[maxn][maxn];

10 int n;

11 int A,B;

12 bool vis[maxn];

13 int step[maxn];

14 int a[maxn];

15 bool flag=true;

16 

17 void bfs(int src)

18 {

19     memset(vis,false,sizeof(vis));

20     queue<int>q;

21     q.push(src);

22     step[src]=0;

23     vis[src]=true;

24     while(!q.empty())

25     {

26         int u=q.front(); q.pop();

27         if(u==B)

28         {

29             flag=false;

30             return ;

31         }

32         int next=u+a[u];

33         if(next>=1&&next<=n&&!vis[next])

34         {

35             step[next]=step[u]+1;

36             vis[next]=true;

37             q.push(next);

38         }

39         next=u-a[u];

40         if(next>=1&&next<=n&&!vis[next])

41         {

42             step[next]=step[u]+1;

43             vis[next]=true;

44             q.push(next);

45         }

46     }

47 }

48 int main()

49 {

50     while(scanf("%d",&n)!=EOF)

51     {

52         if(n==0) break;

53         scanf("%d%d",&A,&B);

54         for(int i=1; i<=n; i++)

55         {

56             cin>>a[i];

57         }

58         flag=true;

59         bfs(A);

60         if(!flag)

61         printf("%d\n",step[B]);

62         else

63         printf("-1\n");

64     }

65     return 0;

66 }

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