Valid Palindrome — LeetCode 125 Python Solution

The problem

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Example

Input

s = "A man, a plan, a canal: Panama"

Output

true

Explanation

"amanaplanacanalpanama" is a palindrome.

Python solution

class Solution:
    def isPalindrome(self, s: str) -> bool:
        i, j = 0, len(s) - 1
        while i < j:
            if not s[i].isalnum():
                i += 1
            elif not s[j].isalnum():
                j -= 1
            elif s[i].lower() != s[j].lower():
                return False
            else:
                i, j = i + 1, j - 1
        return True

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 125. Valid Palindrome is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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