DSA interview question

Check if a String is a Palindrome Using Recursion

Easy · Algorithms · Topics: recursion, string, two pointer

The problem

A word-game app highlights palindromes, words that read the same forwards and backwards, like racecar or abba. The check itself is easy with a loop, but the exercise is to express it recursively: a string is a palindrome if its first and last characters match and the part in between is also a palindrome. This problem shows recursion returning a boolean, where one mismatch anywhere makes the whole answer false and short-circuits the remaining calls.

Example: "racecar" → true, "abc" → false.

Function signature

bool isPalindrome(const string& s, int left, int right);  // call with (s, 0, s.size() - 1)

Test cases

Input `s`	Output	Explanation
`"abba"`	true	Even length
`"racecar"`	true	Odd length; the middle character is its own mirror
`"abc"`	false	'a' ≠ 'c' at the first comparison
`"a"`	true	A single character
`""`	true	The empty string reads the same both ways
`"Racecar"`	false	Case-sensitive: 'R' ≠ 'r'
`"abca"`	false	Ends match, but "bc" in the middle doesn't

(Confirm: comparison is exact and case-sensitive; a variant that ignores case and non-letters is discussed in the follow-ups.)

How interviewers phrase it

"Check whether a string is a palindrome using recursion."
LeetCode 125 variant: "Valid Palindrome", which ignores case and non-alphanumeric characters.
Often paired with "reverse an array using recursion".

Clarifying questions to ask

Case-sensitive or not?
Should spaces and punctuation be ignored? ("A man, a plan, a canal: Panama")
Is recursion required?
How long can the string be? (Recursion depth.)

Follow-ups to expect

"What are the base cases?"
"How would you ignore case and punctuation?"
"Why pass indices instead of substrings?"
"Complexity?"

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