Find All Duplicates

The Negation Mark (O(1) Space)

How do you find duplicates in a list without using extra memory? Since the numbers in our list are restricted to the range 1..N, we can use the numbers themselves as "markers." By flipping the sign of a number at a specific index to negative, we can "remember" that we've seen it before!

The Assignment

Your mission is to find all repeating numbers. Your function receives an array nums of size N.

1. Iterate through the array. For each number, treat its absolute value as an index: idx = Math.abs(nums[i]) - 1.
2. Check the value at that idx:
   • If it is already negative, then the current number is a duplicate!
   • Otherwise, multiply the value at idx by -1 to mark it as "seen."
3. Print every duplicate you find on a new line.