125. Valid Palindrome

Question

LeetCode Problem

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 * 10 5
• s consists only of printable ASCII characters.

Solutions

bool isPalindrome(string s) {
	// two pointer approach:
    // use left and right pointer from two side of the the string
    // left go to right side and find char that is alnum
    // right go to left side and find char that is alnum
    // compare them. if not equal , return false;
    // Edge cases:
    // 1. when string is "" -> directly return true
    // 2. when string is "a" any alnum -> handled by the while loop 
    // 3. when string is not any alnum -> handled by the while loop
    // Time Complexity: O(N)
    // Space Complexity: O(1)
    int n = static_cast<int>(s.size());
    int left = 0;
    int right = n - 1;
    while (left < right) {
      while (left < right && !isalnum(static_cast<char>(s[left])))
        ++left;
      while (left < right && !isalnum(static_cast<char>(s[right])))
        --right;
      if (left < right) {
        if (tolower(s[left]) != tolower(s[right])) {
          return false;
        }
        ++left;
        --right;
      }
    }
    return true;
}