Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int map[n] = {0};
    int max = 0;
    for(int i = 0; i < n; i++){
        map[arr[i]]++;
        // if(arr[i] > ma)
    }
    for(int i = 0; i < 1000; i++){
        if(map[i] >=2 ) return i;
    }

    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