The "Valid Palindrome" interview question is a foundational string problem that asks you to check if a phrase is a palindrome. Crucially, the problem usually specifies that you should ignore non-alphanumeric characters (like spaces, commas, or symbols) and treat uppercase and lowercase letters as identical. A palindrome reads the same forward and backward after these adjustments.
This "Valid Palindrome" coding problem is a staple in interviews at companies like Meta, Amazon, and Microsoft. It tests basic string manipulation skills and the ability to use the "Two Pointers interview pattern" effectively. It’s an entry-level problem that allows interviewers to see if a candidate can write clean, efficient code with minimal space overhead.
The optimal algorithmic pattern is the "Two Pointers" technique. You place one pointer at the start of the string and another at the end. You move the pointers toward the middle, skipping any non-alphanumeric characters along the way. At each step, you compare the characters at the two pointers.
Consider the string: "Race Car!".
!, treat R as r.R, Pointer 2 starts at r.R (lowercase r) matches r. Move pointers.a, Pointer 2 at a. Match. Move.c, Pointer 2 at c. Match. Move.e. Match.A common mistake is creating a new, reversed version of the string, which uses O(n) extra space. While correct, it's less efficient than the two-pointer approach. Another error is failing to correctly identify all non-alphanumeric characters or forgetting to handle the case where the string becomes empty after filtering.
To excel in the "Valid Palindrome" coding problem, practice using built-in language functions to check for alphanumeric characters (like isalnum() in Python or Character.isLetterOrDigit() in Java). This keeps your code concise and reduces the chance of manual errors when filtering the string.