Valid Palindrome Two Pointer in DSA C - Build from Scratch

Valid Palindrome Two Pointer

📖 Scenario: Imagine you are building a simple text checker that tells if a word or phrase reads the same backward as forward, ignoring spaces, punctuation, and letter case. This is called a palindrome.We will use a two-pointer technique to check this efficiently.

🎯 Goal: Build a program that uses two pointers to check if a given string is a palindrome, ignoring non-alphanumeric characters and case differences.

📋 What You'll Learn

Create a string variable with the exact value "A man, a plan, a canal: Panama"

Create two integer variables called left and right to point to the start and end of the string

Implement a loop that moves left and right pointers towards the center, skipping non-alphanumeric characters

Compare characters at left and right ignoring case

Print "true" if the string is a palindrome, otherwise print "false"

💡 Why This Matters

🌍 Real World

Palindrome checks are used in text processing, data validation, and coding challenges.

💼 Career

Understanding two-pointer techniques and string manipulation is important for software development and technical interviews.

1

Create the input string

Create a character array called s and initialize it with the exact string "A man, a plan, a canal: Panama".

DSA C

# Create the string variable s with the given value
// Your code here

Hint

Use char s[] = "A man, a plan, a canal: Panama"; to create the string.

2

Initialize two pointers

Add two integer variables called left and right. Set left to 0 and right to the index of the last character in s (use strlen(s) - 1). Include #include <string.h> at the top.

DSA C

#include 
#include 

int main() {
    char s[] = "A man, a plan, a canal: Panama";
    // Initialize left and right pointers
    // Your code here
    return 0;
}

Hint

Use int left = 0; and int right = strlen(s) - 1; to set pointers.

3

Implement the two-pointer palindrome check

Write a while loop that runs while left < right. Inside the loop, skip non-alphanumeric characters by moving left forward and right backward. Use isalnum() from <ctype.h>. Compare characters at left and right ignoring case using tolower(). If they differ, break the loop. Otherwise, move left forward and right backward.

DSA C

#include 
#include 
#include 

int main() {
    char s[] = "A man, a plan, a canal: Panama";
    int left = 0;
    int right = strlen(s) - 1;
    // Implement two-pointer palindrome check
    // Your code here
    return 0;
}

Hint

Use nested while loops to skip non-alphanumeric characters and compare characters with tolower().

4

Print the result

After the loop, print "true" if the string is a palindrome (use the variable is_palindrome), otherwise print "false". Use printf().

DSA C

#include 
#include 
#include 

int main() {
    char s[] = "A man, a plan, a canal: Panama";
    int left = 0;
    int right = strlen(s) - 1;
    int is_palindrome = 1;

    while (left < right) {
        while (left < right && !isalnum(s[left])) {
            left++;
        }
        while (left < right && !isalnum(s[right])) {
            right--;
        }
        if (tolower(s[left]) != tolower(s[right])) {
            is_palindrome = 0;
            break;
        }
        left++;
        right--;
    }
    // Print "true" if palindrome, else "false"
    // Your code here
    return 0;
}

Hint

Use printf("true\n"); or printf("false\n"); based on is_palindrome.