** Hello I tried to solve question 1949B - Charming Meals , first I used brute force and solved it , later i tried optimising it using binary search but I am getting a wrong answer on 14 testcase , can someone explain me what can be the edge case or whats wrong in my approach **

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

// #include <atcoder/all>

#define ordered_set tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update>
#define int long long


const int mod = 1e9 + 7;
const int N = 1e5 + 9;
const int inf = 1e18;

using namespace __gnu_pbds;
using namespace std;
// using namespace atcoder;



void solve()
{
    
    int n;
    cin >> n;
    vector<int> a(n), b(n);
    for(int i = 0; i < n; i++)
    {
        cin >> a[i];
    }
    for(int i = 0; i < n; i++)
    {
        cin >> b[i];
    }
    sort(a.begin(), a.end());
    sort(b.begin(), b.end());
    int ans = INT_MIN;
    int l = 0, r = n;
   
    while(l<r)
    {
        int mid = (l+r)/2;
        int val1 = INT_MAX;
        int val2 = INT_MAX;
        for(int i = 0; i < n; i++)
        {
            val1 = min(val1, abs(a[i]-b[(i+mid-1)%n]));
            val2 = min(val2, abs(a[i]-b[(i+mid)%n]));
        }
        if(val2 >= val1)
        {
            l = mid + 1;
            ans = max({ans,val1,val2});
        }
        else 
        {
            r = mid;
        }
    }

    // edge case removing attempt

    for(int i=l-10;i<l+10;i++)
    {
        int val = INT_MAX;
        for(int j=0;j<n;j++)
        {
            val = min(val,abs(a[j]-b[abs((j+i))%n]));
        }
        ans = max(ans,val);
    }
    cout << ans << endl;
}

signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int t = 1;
    cin >> t;
    while (t--) {
        solve();
    }
    return 0;
}

1982D - Beauty of the mountains

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
// #include <atcoder/all>

#define ordered_set tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update>
// #define int long long


const int mod = 1e9 + 7;
const int N = 1e5 + 9;
const int inf = 1e9;

using namespace __gnu_pbds;
using namespace std;
// using namespace atcoder;



void solve() {
    int n, m, k;
    cin >> n >> m >> k;
    
    vector<vector<int>> mountain(n, vector<int>(m));
    vector<string> mountain_type(n);
    
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> mountain[i][j];
        }
    }
    
    for (int i = 0; i < n; i++) {
        cin >> mountain_type[i];
    }
    
    int snowy = 0, not_snowy = 0;

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            if (mountain_type[i][j] == '0') {
                not_snowy += mountain[i][j];
            } else {
                snowy += mountain[i][j];
            }
        }
    }

    int diff = abs(snowy - not_snowy);
    
   
    vector<vector<int>> prefix(n + 1, vector<int>(m + 1, 0));

    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= m; j++) {
            prefix[i][j] = (mountain_type[i - 1][j - 1] - '0') + prefix[i - 1][j] + prefix[i][j - 1] - prefix[i - 1][j - 1];
        }
    }
    
    set<int> factors;
    
   
    for (int i = k; i <= n; i++) {
        for (int j = k; j <= m; j++) {
            int total_snowy = prefix[i][j] - prefix[i - k][j] - prefix[i][j - k] + prefix[i - k][j - k];
            factors.insert(abs(k * k - 2 * total_snowy));
        }
    }
    
    int tot_gcd = (factors.size()==0) ? 0 : *factors.begin();
    if(factors.size()==0) {
        if(diff == 0) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
            return;
    }
    for (auto x : factors) {
     if (x == 0) {
            continue;
        }
        tot_gcd = __gcd(tot_gcd, x);
    }
    
    if (diff % tot_gcd == 0) {
        cout << "YES" << endl;
    } else {
        cout << "NO" << endl;
    }
}


signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int t = 1;
    cin >> t;
    while (t--) {
        solve();
    }
    return 0;
}