dfs:

#include <bits/stdc++.h>
using namespace std;

int N, A, B;
int a[201], v[201], ans = INT_MAX, flag;
int b[2] = {1, -1}; //电梯方向上1、下-1
void dfs(int x, int cnt) {
    if(x == B) {
        ans = min(ans, cnt);
        flag = true;
        return;
    }
    for(int i=0; i<2; i++) {
        int nx = x + a[x]*b[i];
        if(nx < 1 || nx > B || v[nx]) continue;
        v[nx] = 1;
        dfs(nx, cnt+1);
        v[nx] = 0;
    }
}

int main() {
    cin>>N>>A>>B;
    for(int i=1; i<=N; i++) cin>>a[i];
    v[A] = 1;
    dfs(A, 0);
    if(flag) cout<<ans<<endl;
    else cout<<-1;
    return 0;
}

bfs:

#include <bits/stdc++.h>
using namespace std;

int N, A, B;
int a[201], v[201];
struct Lift {
    int x; //第几层
    int cnt;//到达这一层按键次数
};
int bfs(){
    queue<Lift> q;
    q.push({A, 0});
    v[A] = true;
    while(!q.empty()) {
        Lift lift = q.front(); q.pop();
        if(lift.x == B) return lift.cnt;
        int up = lift.x + a[lift.x];//往上走
        if(up <= B && v[up] == 0) {
            q.push({up, ++lift.cnt});
            v[up] = true;
        }
        int down = lift.x - a[lift.x];//往下走
        if(down > 0 && v[down] == 0) {
            q.push({down, ++lift.cnt});
            v[down] = true;
        }
    }
    return -1;
}

int main() {
    cin>>N>>A>>B;
    for(int i=1; i<=N; i++) cin>>a[i];
    cout<<bfs();
    return 0;
}