PPractick

The problem

A playlist app has a "reverse order" button, and the exercise is to implement the reversal in place using recursion rather than a loop. The classic approach uses two pointers: swap the first and last elements, then recursively reverse the part in between. It's a clean illustration of how recursion shrinks the problem from both ends, and the same structure reappears in recursive palindrome checks and in divide-and-conquer algorithms.

Example: [1, 2, 3, 4, 5] → [5, 4, 3, 2, 1].

Function signature

void reverseTwoPointers(vector<int>& a, int left, int right);  // call with (a, 0, n - 1)
void reverseOnePointer(vector<int>& a, int i = 0);             // swaps a[i] and a[n - 1 - i]

Test cases

Input a	Output	Explanation
[1, 2, 3, 4, 5]	[5, 4, 3, 2, 1]	The middle element stays put
[1, 2, 3, 4]	[4, 3, 2, 1]	Even length: every element moves
[1, 2]	[2, 1]	One swap
[7]	[7]	Nothing to swap
[]	[]	Empty array

(Confirm: reverse in place; the original vector is modified.)

How interviewers phrase it

• "Reverse an array using recursion."
• "Reverse an array in place" (a loop answer is fine unless recursion is required).
• Often paired with "check whether a string is a palindrome using recursion."

Clarifying questions to ask

1. In place, or return a new array?
2. Is recursion required?
3. How large can the array be? (Recursion depth is about n / 2.)

Follow-ups to expect

1. "What's the base case?"
2. "Can you do it with a single index?"
3. "Recursion depth?"
4. "How would you reverse only part of the array, say indices l to r?"

Related questions

• Check if String is Palindrome or Not Easy
• Remove duplicates from Sorted array Easy
• Move Zeros to End Easy
• Union of two sorted arrays Easy
• Sort an array of 0's 1's and 2's Medium
• Rearrange array elements by sign Medium

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