[LeetCode] 287. Find the Duplicate Number

1. Imagine the array is a linked list. Take the value of each element as a pointer to next element. if there is a duplicate element, there must be a loop in the linked list. Use fast, slow pointer to find the loop. O(n)
2. BitMap - TODO

public class Solution {
    public int findDuplicate(int[] nums) {
        if (nums == null || nums.length == 0) {
            return -1;
        }
        
        int slow = nums[0];
        int fast = nums[nums[0]];
        
        while (slow != fast) {
            slow = nums[slow];
            fast = nums[nums[fast]];
        }
        
        fast = 0;
        while (slow != fast) {
            slow = nums[slow];
            fast = nums[fast];
        }
        
        return slow;
    }
}