Find the Duplicate Number

Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.

• Time: O(n)
• Space: O(1)

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

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