FindDuplicateNumber.java

/*
Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive.

There is only one repeated number in nums, return this repeated number.

You must solve the problem without modifying the array nums and uses only constant extra space.
Follow up:

How can we prove that at least one duplicate number must exist in nums?
Can you solve the problem in linear runtime complexity?
 */
public class FindDuplicateNumber {
    public static void main(String[] args) {
        int[] nums = {1,2,3,4,2};
        System.out.println(findDuplicate(nums));
    }
    public static int findDuplicate(int[] nums) {
        int slow = 0, fast = 0;
        do{
            slow = nums[slow];
            fast = nums[nums[fast]];
        }while(slow != fast);
        slow = 0;
        while(slow != fast){
            slow = nums[slow];
            fast = nums[fast];
        }
        return slow;
    }
}

/*
Time complexity: O(n)
Space Complexity: O(1)
 */