классический бинарный поиск

#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;

int main()
{
  int n = 0, m = 0, left = 0, right = 0, t = 0;

  cin >> n;
  int arr[n];
  for (int i = 0; i<n; i++)
  cin >> arr[i];

  right = n-1;

  cin >> m;
  int ar[m];
  for (int i = 0; i<m; i++)
   {
     cin >> ar[i];
     if (ar[i]<arr[0] || ar[i]>arr[n-1])
     ar[i] = -1;
     else while (left<=right)
      {
        int mid = (left+right)/2;
        if (ar[i] == arr[mid])
        {
          ar[i] = mid+1;
          t++;
          break;
        }
        if (ar[i]<arr[mid])
        right = mid-1;
        else left = mid+1;
      }
      if (t == 0)
      ar[i] = -1;
      t = 0;
      left = 0;
      right = n-1;
    }
   for (int i = 0; i<m; i++)
   cout << ar[i] << endl;
}