Palindrome check

A string is a palindrome if it reads the same in both directions. Compare from the outside in.

The idea

Put one pointer at each end and walk them towards each other. The first mismatch proves it is not a palindrome. If they meet, it is.

Code

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

def is_palindrome(s: str) -> bool:
    l, r = 0, len(s) - 1
    while l < r:
        if s[l] != s[r]: return False
        l += 1; r -= 1
    return True

public static bool IsPalindrome(string s) {
    int l = 0, r = s.Length - 1;
    while (l < r) {
        if (s[l] != s[r]) return false;
        l++; r--;
    }
    return true;
}

Tips

• For "ignore case and punctuation" versions, skip characters where char.IsLetterOrDigit is false and compare with char.ToLowerInvariant.

In the .NET library

s.SequenceEqual(s.Reverse()) is the one-liner. It allocates and compares the full length, where two pointers stop at the first mismatch.