Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {

  for (int i = 0; i < n; ++i) {
    for (int j = i + 1; j >= 0 && j < n; ++j) {
      if (arr[i] == arr[j]) {
        return arr[j];
      }
    }
  }

  return -1;
}

int main() {
  int n;
  scanf("%d", &n);

  int arr[100];
  for (int i = 0; i < n; i++)
    scanf("%d", &arr[i]);

  int result = find_duplicate(arr, n);
  printf("%d", result);
  return 0;
}

Solving Approach