Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>
/*
int find_duplicate(int arr[], int n) {

    int check_value =0;
    int output;
    for(int i = 0; i<n;i++){
        check_value = arr[i];
        for(int j = 0 ; j <n ;j++){
            if( i != j){
                if(check_value == arr[j]){
                    output = arr[j];
                }
            }
        }
    }
    return output;
}
*/

int find_duplicate(int arr[], int n) {
    bool seen[100000] = {0};  // size depends on input range
    for(int i=0;i<n;i++){
        if(seen[arr[i]]) return arr[i]; // first duplicate
        seen[arr[i]] = true;
    }
    return -1; // no duplicate
}

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