Solve Python Challenges: Reverse String, Palindrome, Fibonacci, Unique List

Scenario

Live coding round – four quick Python exercises

Question

Implement a function that reverses a string in-place. Write code that returns True if a given integer is a palindrome, False otherwise. Generate the n-th Fibonacci number iteratively without recursion. Given an unsorted list of integers, return a new list containing only the unique values, preserving original order.

Hints

Each task should run in O(n) time or better, and you may not import external libraries.

Unique Elements Preserving Original Order

Given an unsorted list of integers nums, return a new list containing each distinct value exactly once, preserving the order of their first occurrence. Do not sort or otherwise reorder the input.

Constraints

• 0 <= len(nums) <= 200000
• -1000000000 <= nums[i] <= 1000000000
• Preserve the order of first occurrences
• Do not sort the input
• Target time complexity: O(n)
• Target space complexity: O(n)
• Do not import external libraries

Examples

Example 1

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

Explanation: Keep the first occurrence of each number: 1, then 2, then 3, then 4.

Example 2

Input: nums = [-1, -1, -2, -1, -2, -3]
Output: [-1, -2, -3]

Explanation: Preserve order of first appearances: -1, then -2, then -3.

Hints

1. Use a hash set to track which values have been seen.
2. Append a number to the result only the first time it appears.
3. Avoid sorting; membership checks in a set are O(1) on average.

Approach

Iterate once over nums, maintaining a set of seen values. For each element, if it is not in seen, add it to both seen and the result list. This preserves the order of first occurrences while removing duplicates.

• Time complexity: O(n)
• Space complexity: O(n)