$T(n) = O(n), S(n) = O(1)$

class Solution {
    public int duplicateInArray(int[] nums) {
        int len = nums.length;
        for (int num : nums) {
            if (num < 0 || num >= len) {
                return -1;
            }
        }

        for (int i = 0; i < len; i++) {
            while (nums[i] != i) {
                if (nums[i] == nums[nums[i]]) {
                    return nums[i];
                }
                int temp = nums[i];
                nums[i] = nums[temp];
                nums[temp] = temp;
            }
        }

        return -1;
    }
}