Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    // Your logic here
    int *pFirst = arr; 
    int *pNext = pFirst+1; 
    while (1){
        if ((*pFirst)-(*pNext) !=0){
            ++pNext; 
            if (pNext > pFirst+(n-1)){
                ++pFirst; 
                pNext = pFirst+1; 
            }
        }else {
            return *pFirst; 
        }
    }
    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