125. Valid Palindrome

📋 Đề Bài

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.

🧠 Thuật Toán & Kỹ Thuật

String (Chuỗi)

⏱️ Thời gian O(n)

💾 Không gian O(1)

💻 Lời Giải

C++ 0125-valid-palindrome.cpp

class Solution {
public:
    bool isPalindrome(string s) {
        string t;
        
        for (char c : s) {
            c = tolower(c);
            if ('a' <= c and c <= 'z') {
                t += c;
            }
            else if ('0' <= c and c <= '9') {
                t += c;
            }
        }
        
        return t == string(t.rbegin(), t.rend());
    }
};