intro practice · coding problem
Valid palindrome with two pointers
Given a string, decide if it reads the same forwards and backwards, ignoring case and non-alphanumeric characters. Use two pointers, no extra string.
The mechanism
Two pointers replace a nested loop with a single pass by exploiting structure — usually a sorted array or a sequence with a monotonic property. The pointers move toward or away from each other based on a comparison, so each element is visited once.
Expected evidence
isPalindrome('A man, a plan, a canal: Panama') -> trueTry this drill in the practice workspace →
Hints when you need them
- Start one pointer at each end, move inward.
- Skip non-alphanumeric characters without allocating a cleaned copy.
Watch for these mistakes
- Using two pointers on unsorted data when the technique needs order
- Off-by-one errors at the start/end conditions
- Moving the wrong pointer and skipping valid pairs