数组中重复的数字

Input:
{2, 3, 1, 0, 2, 5}

Output:
2

public int duplicate(int[] nums) {
    for (int i = 0; i < nums.length; i++) {
        while (nums[i] != i) {
            if (nums[i] == nums[nums[i]]) {
                return  nums[i];
            }
            swap(nums, i, nums[i]);
        }
    }
    return -1;
}

private void swap(int[] nums, int i, int j) {
    int t = nums[i];
    nums[i] = nums[j];
    nums[j] = t;
}