超短解答

bool duplicate(int A[], int len, int* dup) {
       for(int i = 0; i < len; i++){
            while(A[i] != i){
                if(A[A[i]] == A[i]){
                    *dup = A[i];
                    return true;
                }else{
                    std::swap(A[i], A[A[i]]);
                }
            }
        }
        return false;
    }