Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int mxd=0;
    for(int i=0;i<n;i++){
        mxd^=arr[i]^i;
    }
    int rlb=mxd & -mxd;
    int cand1=0,cand2=0;
    for(int i=0;i<n;i++){
        arr[i] & rlb ? cand1^=arr[i] : cand2^=arr[i];
        i & rlb ? cand1^=i : cand2^=i;
    }
    int cnt=0;
    for(int i=0;i<n;i++) if(cand1==arr[i]) cnt++;
    return cnt==2 ? cand1 : cand2;
}

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