Valid Palindrome: converting uppercase to lowercase and removing non-alphanumeric characters
Valid Palindrome: converting uppercase to lowercase and removing non-alphanumeric characters
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
For example,
“A man, a plan, a canal: Panama” is a palindrome. “race a car” is not a palindrome.
Example Questions Candidate Might Ask:
Q: What about an empty string? Is it a valid palindrome?
A: For the purpose of this problem, we define empty string as 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.
O(n) runtime, O(1) space:
The idea is simple, have two pointers – one at the head while the other one at the tail. Move them towards each other until they meet while skipping non-alphanumeric characters.
Consider the case where given string contains only non-alphanumeric characters. This is a valid palindrome because the empty string is also a valid palindrome.
Solution in Java
public boolean isPalindrome(String s) {
int i = 0, j = s.length() - 1;
while (i < j) {
while (i < j && !Character.isLetterOrDigit(s.charAt(i))) i++;
while (i < j && !Character.isLetterOrDigit(s.charAt(j))) j--;
if (Character.toLowerCase(s.charAt(i))
!= Character.toLowerCase(s.charAt(j))) {
return false;
}
i++; j--; }
return true;
}
Solution in Python
class Solution(object): def isPalindrome(self, s): """ :type s: str :rtype: bool """ alnum_s = [t.lower() for t in s if t.isalnum()] ls = len(alnum_s) if ls <= 1: return True mid = ls / 2 for i in range(mid): if alnum_s[i] != alnum_s[ls - 1 - i]: return False return True