思路

此题是一张有向图，我们先建图，再bfs，记得加上记忆化。

AC Code

#include<bits/stdc++.h>
#define ll long long
using namespace std;
const ll N=205;
ll n,A,B,ans=-1;
struct node1{
    ll up,down;
}a[N];
struct node2{
    ll x,cnt;
};
bool flag[N];
queue<node2> q;
void bfs(ll x){
    q.push({x,0});
    while(!q.empty()){
        node2 p=q.front();
        q.pop();
        if(p.x<1||p.x>n||flag[p.x])
            continue;
        flag[p.x]=1;
        if(p.x==B){
            ans=p.cnt;
            return;
        }
        q.push({a[p.x].up,p.cnt+1}),q.push({a[p.x].down,p.cnt+1});
    }
}
int main(){
    scanf("%lld%lld%lld",&n,&A,&B);
    for(ll i=1,x;i<=n;i++)
        scanf("%lld",&x),a[i].up=i+x,a[i].down=i-x;
    bfs(A);
    printf("%lld",ans);
    return 0;
}