Palindrom String

Write a recursive function is_palindrome(s) that returns True if the string reads the same forward and backward, and False otherwise. Input: "racecar" → True. Input: "hello" → False. You must use recursion — no for/while loops allowed.

This combines the recursive thinking from this week with the two-pointer palindrome logic from Week 1. Instead of using a while loop with left/right pointers, we recurse: check if the outermost characters match, then recursively check the substring between them. The base case is when the pointers cross (a single character or empty string is always a palindrome).

1. Pass left and right pointer indices along with the string. 2. Base case: if left >= right, return True (we have checked all pairs). 3. If s[left] != s[right], return False immediately. 4. Otherwise, recurse with left + 1 and right - 1.

- Not passing the left and right pointers, trying to slice the string instead (creates unnecessary copies) - Using left > right as the base case instead of left >= right (misses the single-character case) - Forgetting to handle case sensitivity (if required by the problem) - Creating new string slices at each level, which adds O(N) per call

This problem beautifully demonstrates how the same algorithm (two-pointer palindrome check) can be expressed both iteratively and recursively. The iterative version is more space-efficient (O(1) vs O(N)), but the recursive version reinforces the pattern of "check the boundaries, then recurse inward." This same recursive shrinking appears in binary search and divide-and-conquer.