使用bfs(广度优先搜索)解题即可
复杂度O(n)

#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
int n,s,t,a[201],cmp[201],val[201];
int main(){
    cin>>n>>s>>t;
    int i;
    for(i=1;i<=n;i++){
        cin>>a[i];
        cmp[i]=0;
        val[i]=-1;
    }
    if(s==t) {
     cout<<0;return 0;
    }
    int l,r,v,qu[1001];
    l=r=1;
    qu[1]=s;
    val[s]=0;
    cmp[s]=1;//cmp标记已走过楼层,因为正确路线不能重复 
    while(l<=r && r<=1000){
        v=qu[l++];
        if(v+a[v]<=n) if(cmp[v+a[v]]==0){
            qu[++r]=v+a[v];
            val[v+a[v]]=val[v]+1;
            cmp[v+a[v]]=1;
        }
        if(v-a[v]>=1) if(cmp[v-a[v]]==0){
            qu[++r]=v-a[v];
            val[v-a[v]]=val[v]+1;
            cmp[v-a[v]]=1;
        }
    }
    cout<<val[t];
    return 0;
}