[Oracle] LeetCode 1802 Maximum Value at a Given Index in a Bounded Array

class Solution {
private:
    #define ll long long
    bool check(ll val, int n, int idx, ll maxSum){
        int N_left_to_idx = idx;
        int N_right_to_end = (n-1)-(idx+1)+1;
        
        int AP_series = val-1; // from 1 to val-1
        int N_left_ones = 0, N_right_ones = 0;
        ll leftsum=0, rightsum=0;
        
        if(N_left_to_idx>=AP_series){
            // can contain the whole AP series
            leftsum = (val-1)*val/2;
            N_left_ones = N_left_to_idx - AP_series;
        }
        else{
            // cannot contain the whole AP series
            // AP starts from val-1 to val-idx
            leftsum = N_left_to_idx * (val-1+val-idx)/2;
        }
        
        if(N_right_to_end >= AP_series){
            rightsum = (val-1)*val/2;
            N_right_ones = N_right_to_end - AP_series;
        }
        else{
            rightsum = N_right_to_end*(val-1+val-N_right_to_end)/2;
        }
        return rightsum+leftsum+N_left_ones+N_right_ones+val<=maxSum;
        
    }
public:
    int maxValue(int n, int index, ll maxSum) {
        ll l = 1, r = maxSum;
        int ans=0;
        while(l<=r){
            ll mid = (r-l)/2+l;
            if(check(mid, n, index, maxSum)){
                ans=mid; l=mid+1;
            }
            else r=mid-1;
        }
        return ans;
    }
};