花花酱 LeetCode 125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

Solution: Two pointers

Time complexity: O(n)
Space complexity: O(1)

C++

class Solution {

public:

bool isPalindrome(string s) {

int i = 0;

int j = s.length() - 1;

while (i < j) {

while (i < j && !isalnum(s[i])) ++i;

while (i < j && !isalnum(s[j])) --j;

if (tolower(s[i++]) != tolower(s[j--]))

return false;

}

return true;

}

};

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