The provided code defines a C++ class `Solution` with a...

August 23, 2025 at 07:07 AM

class Solution { public: bool isPalindrome(string s) { // Remove all non-alpha chars // Shuffle them to the front, why? auto newEnd = std::remove_if(s.begin(), s.end(), [](unsigned char c) { return !std::isalnum(c); }); // Removes all non-alpha chars s.erase(newEnd, s.end()); // Convert to lowercase std::transform(s.begin(), s.end(), s.begin(), [](unsigned char c) { return std::tolower(c); }); // check if it's palindrome int r = s.size()-1; int l = 0; while (l < r) { if (s[l] != s[r]) { return false; } l++; r--; } return true; } };

The provided code defines a C++ class Solution with a method isPalindrome that checks if a given string s is a palindrome. A palindrome is a string that reads the same backward as forward, ignoring case and non-alphanumeric characters.

Here’s a step-by-step explanation of what the code does:

  1. Remove Non-Alphanumeric Characters:

    • The line using std::remove_if identifies and "removes" all characters in the string s that are not alphanumeric (letters or digits) using the condition !std::isalnum(c).
    • Technically, std::remove_if shifts non-removed elements to the front of the string and returns an iterator pointing to the new logical end (position after the last valid element).
    • The s.erase(newEnd, s.end()) call then truncates the string, effectively deleting the unwanted non-alphanumeric characters from it.

  2. Convert All Characters to Lowercase:

    • The std::transform call goes through the cleaned-up string s and transforms all its characters to lowercase using std::tolower.
    • This ensures that the palindrome check is case-insensitive.

  3. Check if the String is a Palindrome:

    • Two pointers, l (left) and r (right), are initialized to the start and end of the string.
    • A while loop runs to compare characters at these two positions. If at any point the characters differ, the method returns false.
    • If the loop completes without mismatches, the string is a palindrome, and the method returns true.

Example Usage:

Given input:

  • "A man, a plan, a canal: Panama" ➝ The method:

    1. Strips non-alphanumeric characters: "AmanaplanacanalPanama".
    2. Converts to lowercase: "amanaplanacanalpanama".
    3. Checks for palindrome: Matches, so returns true.

  • "race a car" ➝ After cleaning: "raceacar", not a palindrome, so returns false.

Summary:

The isPalindrome method checks if a given string is a palindrome while ignoring case sensitivity and non-alphanumeric characters. It uses std::remove_if to filter characters, std::transform to normalize case, and a two-pointer technique to compare characters.

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