Is Valid Palindrome II easy or hard?

LeetCode classifies Valid Palindrome II as an Easy problem. The difficulty mainly comes from recognizing that only one deletion is allowed and handling the first mismatch correctly using a two-pointer check.

How to solve Valid Palindrome II in O(n)?

Use two pointers starting at the beginning and end of the string. Move inward while characters match. On the first mismatch, check if either substring formed by skipping one character is a palindrome. Since only one deletion is allowed, the process stays linear with O(n) time.

What is the best approach for Valid Palindrome II?

The two pointer approach is the best solution. Start with pointers at both ends of the string and move inward while characters match. When a mismatch appears, check whether skipping either the left or right character produces a palindrome. This runs in O(n) time with O(1) extra space.

What data structure is used in Valid Palindrome II?

The problem primarily uses the two pointers technique on a string. No additional data structures such as hash maps or stacks are required because the palindrome property can be verified by comparing characters from both ends.

What is the time complexity of Valid Palindrome II?

The optimal solution runs in O(n) time where n is the length of the string. Each character is compared at most once during the two-pointer scan, and a mismatch triggers at most one additional linear palindrome check. Space complexity is O(1) for the iterative approach.

Valid Palindrome II Python or Java solution approach?

Both Python and Java implementations typically follow the same two-pointer strategy. Maintain left and right indices, compare characters, and on mismatch call a helper function that checks if the remaining substring is a palindrome. The logic remains O(n) time and O(1) space.

Is Valid Palindrome II asked at Google, Amazon, or Meta?

Valid Palindrome II appears frequently in coding interviews because it tests the two pointers pattern and edge case handling. Similar string symmetry problems have been reported in interviews at companies like Amazon, Google, and Meta.

680. Valid Palindrome II - Practice Online

Given a string s, return true if the s can be palindrome after deleting at most one character from it.

Example 1:

Input: s = "aba"
Output: true

Example 2:

Input: s = "abca"
Output: true
Explanation: You could delete the character 'c'.

Example 3:

Input: s = "abc"
Output: false

Constraints:

1 <= s.length <= 10⁵
s consists of lowercase English letters.