[LeetCode - 125] Valid Palindrome - C++

문제

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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.

 

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

 

풀이

class Solution {
public:
    bool isPalindrome(string s) {
        int l = 0, r = s.size() - 1;
        while(l < r) {
            while(l < r && !isAlphanumeric(s[l])) l++;
            while(l < r && !isAlphanumeric(s[r])) r--;
            if(tolower(s[l]) != tolower(s[r])) return false;
            l++; r--;
        }
        return true;
    }

    bool isAlphanumeric(char c) {
        return (
            'A' <= c && c <= 'Z' ||
            'a' <= c && c <= 'z' ||
            '0' <= c && c <= '9'
        );
    }
};

투포인터가 양쪽에서 가운데로 모이며 non-alphanumeric 문자는 무시한다.