// Runtime: 0 ms, faster than 100.00% of Java online submissions for Find the Duplicate Number.
class Solution {
    public int findDuplicate(int[] nums) {
        int slow, normal, fast;
        slow = normal = fast = 0;

        do { normal = nums[normal]; fast = nums[nums[fast]]; } while (normal != fast);
        do { slow = nums[slow]; normal = nums[normal]; } while (slow != normal);

        return normal;
    }
}