int Solution::jump(vector<int> &a) {
    int n=a.size();
    vector <bool> visited(n,false);
    visited[0]=true;
    queue <int> q;
    q.push(0);
    int ans=0;
    while(!q.empty())
    {
        int n1=q.size();
        for(int i=0;i<n1;i++)
        {
            int s=q.front();
            if(s==n-1)
                return ans;
            q.pop();
            for(int j=s;j<=min(s+a[s],n-1);j++)
            {
                if(!visited[j])
                {
                    visited[j]=true;
                    q.push(j);
                }
            }
        }
        ans++;
    }
    return -1;
}