剑指offer读书笔记之找出数组中重复的数字

public class  {
    private static int findDuplicateNum(int[] arr) {
        if (arr == null || arr.length == 0) {
            return -1;
        }
        for (int i = 0; i < arr.length; i++) {
            while (arr[i] != i) {
                if (arr[i] == arr[arr[i]]) {
                    return arr[i];
                } else {
                    int temp = arr[i];
                    arr[i] = temp;
                    arr[temp] = temp;
                }
            }
        }
        return -1;
    }
    public static void main(String[] args) {
        int[] arr = {2, 3, 1, 0, 2, 5, 3};
        System.out.println(findDuplicateNum(arr));
    }
}