Kth element in Matrix

//{ Driver Code Starts
// kth largest element in a 2d array sorted row-wise and column-wise
#include<bits/stdc++.h>
using namespace std;
#define MAX 1000
int mat[MAX][MAX];
int kthSmallest(int mat[MAX][MAX], int n, int k);
// driver program to test above function
int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        int n;
        cin>>n;

        for(int i=0;i<n;i++)
            for(int j=0;j<n;j++)
                cin>>mat[i][j];
        int r;
        cin>>r;
        cout<<kthSmallest(mat,n,r)<<endl;
    }
     // cout << "7th smallest element is " << kthSmallest(mat, 4, 7);
      return 0;
}

// } Driver Code Ends



int kthSmallest(int mat[MAX][MAX], int n, int k)
{
  //Your code here
  int min = 1, max =10000, mid, pos=0;
  int p = std::min(k, n);


  while(min<max)
  {
      mid = (min+max)/2;

      pos=0;
      for(int i=0;i<p;i++)
        pos += upper_bound(mat[i], mat[i] + n, mid) - mat[i];

      if(pos<k)
        min = mid+1;

      else max = mid;
  }

  return min;
}