C Program to Check if a Given String is a Palindrome?

A palindrome is a word, number, phrase, or other sequence of characters which reads the same backward as forward. Words such as "madam" or "racecar" or the number "10801" are palindromes.

To check if a string is a palindrome, we need to compare characters from both ends moving toward the center. If the first character matches the last, second matches second-last, and so on, then the string is a palindrome.

Syntax

int isPalindrome(char str[]);
// Returns 1 if palindrome, 0 otherwise

Example 1: Using Character Comparison

This approach compares characters from both ends of the string −

#include <stdio.h>
#include <string.h>

int isPalindrome(char str[]) {
    int left = 0;
    int right = strlen(str) - 1;
    
    while (left < right) {
        if (str[left] != str[right]) {
            return 0; // Not a palindrome
        }
        left++;
        right--;
    }
    return 1; // Is a palindrome
}

int main() {
    char str1[] = "naman";
    char str2[] = "hello";
    
    if (isPalindrome(str1)) {
        printf("%s is a palindrome<br>", str1);
    } else {
        printf("%s is not a palindrome<br>", str1);
    }
    
    if (isPalindrome(str2)) {
        printf("%s is a palindrome<br>", str2);
    } else {
        printf("%s is not a palindrome<br>", str2);
    }
    
    return 0;
}

naman is a palindrome
hello is not a palindrome

Example 2: Case-Insensitive Palindrome Check

This example handles both uppercase and lowercase characters −

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int isPalindromeIgnoreCase(char str[]) {
    int left = 0;
    int right = strlen(str) - 1;
    
    while (left < right) {
        if (tolower(str[left]) != tolower(str[right])) {
            return 0;
        }
        left++;
        right--;
    }
    return 1;
}

int main() {
    char str[] = "Madam";
    
    if (isPalindromeIgnoreCase(str)) {
        printf("%s is a palindrome (ignoring case)<br>", str);
    } else {
        printf("%s is not a palindrome<br>", str);
    }
    
    return 0;
}

Madam is a palindrome (ignoring case)

Key Points

• The basic approach has O(n/2) time complexity and O(1) space complexity.
• The algorithm only needs to check half the string length.
• Use tolower() function for case-insensitive comparison.
• Empty strings and single characters are considered palindromes.

Conclusion

Palindrome checking in C can be efficiently done using the two-pointer technique. This approach compares characters from both ends moving inward, providing optimal time and space complexity for the problem.