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#include <stdio.h>
#include <deque>
#include <string.h>
using namespace std;
 
deque<int>q;
int dp[4000000];
int floor[110000];
int visit[1100];
 
int main( )
{
  int N, i, t, v,s ,e, flag;
  while (scanf("%d",&N), N)
{ 
    flag = 0;
    q.clear();
   scanf("%d%d",&s,&e);
   memset(dp, 0, sizeof(dp));
   memset(visit,0,sizeof(visit));
    for (i = 1; i <= N; i++)
    scanf("%d",&floor[i]);
    q.push_back(s);
     visit[s] = 1;
  while ( !q.empty( ))
  {
     t = q.front( );
    if (t == e) {
    printf("%d\n",dp[t]);
        flag = 1;
        break;
     }
      v =  t + floor[t];
    if ( v >= 1 && v <= N  && !visit[v]) {
    q.push_back(v);
    dp[v] = dp[t]  + 1;
        visit[v] = 1;
     }
     v = t - floor[t];
    if ( v >= 1 && v <= N && !visit[v]) {
    q.push_back(v);
       dp[v] = dp[t]  + 1;
        visit[v] = 1;
     }
      q.pop_front( );
}
 if (!flag)
    puts("-1");
}
  return 0;
}

引自：ic买卖