HackerHeap

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.

Example 1:

Input: [1,3,4,2,2]
Output: 2

Example 2:

Input: [3,1,3,4,2]
Output: 3

Note:

1. You must not modify the array (assume the array is read only).
2. You must use only constant, O(1) extra space.
3. Your runtime complexity should be less than O(n²).
4. There is only one duplicate number in the array, but it could be repeated more than once.

Solution: We will solve this using Two pointers the brute force solution would be looping twice to find the duplicate which will take O(n²).

class HackerHeap {
    public int findDuplicate(int[] nums) {
        int slow = 0;
        int fast = 0;  
        do{
            slow = nums[slow];
            fast = nums[nums[fast]];
        }while(slow!=fast);
        
        int find =0;
        
        while(find!=slow){
            find = nums[find];
            slow = nums[slow];
        }  
        return find;
    }
}