[leetcode] 125. Valid Palindrome _ Algorithm Problem Solve for python

1. Problem

125. Valid Palindrome

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.

Given a string s, return true if it is a palindrome, or false otherwise.

Example 1:

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.


  • 1 <= s.length <= 2 * 10^5
  • s consists only of printable ASCII characters.

2. Solution

I solve this problem like this.

  • Complexity
    • Time complexity : O(N)
    • Space complexity : O(N)
  • Step
    • Using filter and isalnum function, we can easily get alphabet and numbers.
    • And check list value.
class Solution:
    def isPalindrome(self, s: str) -> bool:
        alpha_list = list(filter(str.isalnum, s.lower()))
        for i in range(len(alpha_list)//2):
            if alpha_list[i] != alpha_list[-(i+1)]:
                return False

        return True