Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>
#include <math.h>
int find_duplicate(int arr[], int n) {
    // Your logic here
    int res;
    
    for(int i=0; i<n; i++)
    { 
        if(arr[abs(arr[i])]<0)
        res =abs(arr[i]);
        arr[arr[i]] *= -1;
    }
    return res;
}

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