main.cpp

// Source: https://leetcode.com/problems/valid-palindrome
// Title: Valid Palindrome
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <cctype>
#include <string>

using namespace std;

// Two Pointer
class Solution {
 public:
  bool isPalindrome(const string &s) {
    const int n = s.size();

    for (int i = 0, j = n - 1; i < j; ++i, --j) {
      while (i < n && !isalnum(s[i])) ++i;
      while (j >= 0 && !isalnum(s[j])) --j;
      if (i >= j) break;
      if (tolower(s[i]) != tolower(s[j])) return false;
    }

    return true;
  }
};