Sentence Palindrome

Try it on GfG Practice

Given a sentence s, determine whether it is a palindrome sentence or not. A palindrome sentence is a sequence of characters that reads the same forward and backward after:

• Converting all uppercase letters to lowercase.
• Removing all non-alphanumeric characters (i.e., ignore spaces, punctuation, and symbols).

Examples: 

Input: s = "Too hot to hoot."
Output: true
Explanation: If we remove all non-alphanumeric characters and convert all uppercase letters to lowercase, string s will become "toohottohoot" which is a palindrome.

Input: s = "Abc 012..## 10cbA"
Output: true
Explanation: If we remove all non-alphanumeric characters and convert all uppercase letters to lowercase, string s will become "abc01210cba" which is a palindrome.

Input: s = "ABC $. def01ASDF.."
Output: false
Explanation: If we remove all non-alphanumeric characters and convert all uppercase letters to lowercase, string s will become "abcdef01asdf" which is not a palindrome.

[Naive Approach] Normalization and Reverse Check - O(n) Time and O(n) Space

The idea is to preprocess the given sentence by filtering out all non-alphanumeric characters and converting all uppercase letters to lowercase. This ensures that the comparison is not affected by spaces, punctuation, or letter casing. Once the string is normalized, we simply compare it with its reverse. If both are identical, the sentence is a palindrome; otherwise, it is not.

#include <algorithm>
#include <iostream>
using namespace std;

bool isPalinSent(string &s) {

    // create a new string having only
    // alphanumeric characters
    string s1 = "";
    for (char ch : s) {
        if (isalnum(ch)) {
            s1.push_back(tolower(ch));
        }
    }

    // find reverse of this new string
    string rev = s1;
    reverse(rev.begin(), rev.end());

    // compare string and its reverse
    return s1 == rev;
}

int main() {
    string s = "Too hot to hoot.";

    cout << (isPalinSent(s) ? "true" : "false") << endl;
    return 0;
}

class GfG {
     static boolean isPalinSent(String s) {
  
        // create a new string having only 
        // alphanumeric characters
        StringBuilder s1 = new StringBuilder();
        for (char ch : s.toCharArray()) {
            if (Character.isLetterOrDigit(ch)) {
                s1.append(Character.toLowerCase(ch));
            }
        }
  
        // find reverse of this new string
        StringBuilder rev = new 
                StringBuilder(s1.toString());
        rev.reverse();
  
        // compare string and its reverse
        return s1.toString().equals(rev.toString());
    }

    public static void main(String[] args) {
        
        String s = "Too hot to hoot.";
        System.out.println(isPalinSent(s)
                           			? "true" : "false");
    }
}

def isPalinSent(s):
    # create a new list having only 
    # alphanumeric characters
    s1 = []
    for ch in s:
        if ch.isalnum():
            s1.append(ch.lower())
	
    # convert the new list to string
    s1 = ''.join(s1)
    
    # find reverse of this new string
    rev = s1[::-1]
    
    # compare string and its reverse
    return s1 == rev

if __name__ == "__main__":
    s = "Too hot to hoot."
    print("true" if isPalinSent(s) else "false")

using System;
using System.Text;
using System.Linq;

class GfG {
    static bool isPalinSent(string s) {
        StringBuilder s1 = new StringBuilder();
        
        // Filter out non-alphanumeric characters and 
        // convert to lowercase
        foreach (char ch in s) {
            if (Char.IsLetterOrDigit(ch)) 
                s1.Append(Char.ToLower(ch));
        }
        
        string s2 = s1.ToString();
      
  		// find reverse of this new string
    	char[] revArray = 
    	        s2.ToCharArray().Reverse().ToArray();
    	string rev = new string(revArray);
  	
  		// compare string and its reverse
    	return s2 == rev;
    }

    static void Main() {
        string s = "Too hot to hoot.";
        Console.WriteLine(isPalinSent(s) ? "true" : "false");
    }
}

function isPalinSent(s) {
    // create an array to store only 
    // alphanumeric characters
    let s1 = [];
    for (let i = 0; i < s.length; i++) {
        let ch = s[i];

        // Check if the character is alphanumeric
        if (/[a-zA-Z0-9]/.test(ch)) {
            s1.push(ch.toLowerCase());
        }
    }
    // find reverse of this new string by 
    // joining the array into a string
    let rev = s1.slice().reverse().join("");

    // compare string and its reverse
    return s1.join("") === rev;
}

// Driver Code
let s = "Too hot to hoot.";
console.log(isPalinSent(s) ? "true" : "false");

true

[Expected Approach] Using Two Pointers - O(n) Time and O(1) Space

The idea is to use the two-pointer approach to check if a sentence is a palindrome. We place one pointer at the start and the other at the end of the string, moving them toward each other while comparing characters.
We skip any non-alphanumeric characters and convert uppercase letters to lowercase to ensure a case-insensitive comparison. If the characters at both pointers don't match, we return false. If the pointers cross without a mismatch, the sentence is a palindrome.

#include <iostream>
using namespace std;

bool isPalinSent(string s) {
    int i = 0, j = s.length() - 1;

    // Compares character until they are equal
    while (i < j) {

        // Skip non alphabet character
        // from left side
        if (!isalnum(s[i]))
            i++;

        // Skip non alphabet character
        // from right side
        else if (!isalnum(s[j]))
            j--;

        // If characters are equal
        // update the pointers
        else if (tolower(s[i]) == tolower(s[j]))
            i++, j--;

        // If characters are not equal then
        // sentence is not palindrome
        else
            return false;
    }

    return true;
}

int main() {
    string s = "Too hot to hoot.";

    cout << (isPalinSent(s) ? "true" : "false") << endl;
    return 0;
}

#include <stdio.h>
#include <stdbool.h>

bool isPalinSent(const char* s) {
    int i = 0, j = strlen(s) - 1;

    // Compares character until they are equal
    while (i < j) {

        // Skip non alphabet character
        // from left side
        if (!isalnum(s[i]))
            i++;

        // Skip non alphabet character
        // from right side
        else if (!isalnum(s[j]))
            j--;

        // If characters are equal 
        // update the pointers
        else if (tolower(s[i]) == tolower(s[j]))
            i++, j--;

        // If characters are not equal then
        // sentence is not palindrome
        else
            return false;
    }

    return true;
}

int main() {
    const char* s = "Too hot to hoot.";
    printf("%s\n", isPalinSent(s) ? "true" : "false");
    return 0;
}

class GfG {

    static boolean isPalinSent(String s) {
        int i = 0, j = s.length() - 1;

        // Compares character until they are equal
        while (i < j) {

            // Skip non alphabet character from left side
            if (!Character.isLetterOrDigit(s.charAt(i)))
                i++;

            // Skip non alphabet character from right side
            else if (!Character.isLetterOrDigit(s.charAt(j)))
                j--;

            // If characters are equal update the pointers
            else if (Character.toLowerCase(s.charAt(i)) 
                    == Character.toLowerCase(s.charAt(j))) {
                i++;
                j--;
            }
          
            // If characters are not equal then sentence
            // is not palindrome
            else
                return false;
        }

        return true;
    }

    public static void main(String[] args) {
        String s = "Too hot to hoot.";
        System.out.println(isPalinSent(s) 
                           ? "true" : "false");
    }
}

def isPalinSent(s):
    i, j = 0, len(s) - 1

    # Compares character until they are equal
    while i < j:

        # Skip non alphabet character
        # from left side
        if not s[i].isalnum():
            i += 1

        # Skip non alphabet character
        # from right side
        elif not s[j].isalnum():
            j -= 1

        # If characters are equal 
        # update the pointers
        elif s[i].lower() == s[j].lower():
            i += 1
            j -= 1

        # If characters are not equal then
        # sentence is not palindrome
        else:
            return False

    return True

if __name__ == "__main__":
    s = "Too hot to hoot."
    print("true" if isPalinSent(s) else "false")

using System;

class GfG {

    static bool isPalinSent(string s) {
        int i = 0, j = s.Length - 1;

        // Compares character until they are equal
        while (i < j) {

            // Skip non alphabet character
            // from left side
            if (!char.IsLetterOrDigit(s[i]))
                i++;

            // Skip non alphabet character
            // from right side
            else if (!char.IsLetterOrDigit(s[j]))
                j--;

            // If characters are equal 
            // update the pointers
            else if (char.ToLower(s[i]) == char.ToLower(s[j])) {
                i++;
                j--;
            }

            // If characters are not equal then
            // sentence is not palindrome
            else
                return false;
        }

        return true;
    }

    static void Main() {
        string s = "Too hot to hoot.";
        Console.WriteLine(isPalinSent(s) ? "true" : "false");
    }
}

function isPalinSent(s) {
    let i = 0, j = s.length - 1;

    // Compares character until they are equal
    while (i < j) {

        // Skip non alphabet character
        // from left side
        if (!s[i].match(/[a-zA-Z0-9]/))
            i++;

        // Skip non alphabet character
        // from right side
        else if (!s[j].match(/[a-zA-Z0-9]/))
            j--;

        // If characters are equal
        // update the pointers
        else if (s[i].toLowerCase() === s[j].toLowerCase())
            i++, j--;

        // If characters are not equal then
        // sentence is not palindrome
        else
            return false;
    }

    return true;
}

// Driver Code
let s = "Too hot to hoot.";
console.log(isPalinSent(s) ? "true" : "false");

true

Palindrome Sentence

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