287. Find the Duplicate Number (https://leetcode.com/problems/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.

287. Find the Duplicate Number (#1 Two pointer).java

// Two pointer solution: slow/fast pointer
// Time: O(n), 0ms
// Space: O(1), 36.8mb
class Solution {
    public int findDuplicate(int[] nums) {
        // Treat the nums as a singly linked list
        // The duplicate num is where the cycle begins
        int slow = 0, fast = 0;
        while(true) {
            slow = nums[slow];
            fast = nums[nums[fast]];
            if(slow == fast) break;
        }
        // Find where the cycle begins
        slow = 0;
        while(slow != fast) {
            slow = nums[slow];
            fast = nums[fast];
        }
        return slow;
    }
}