C Program to Check if a Given String is Palindrome

A string is said to be palindrome if the reverse of the string is the same as the string. For example, “abba” is a palindrome because the reverse of “abba” will be equal to “abba” so both of these strings are equal and are said to be a palindrome, but “abbc” is not a palindrome.

There are multiple methods to check palindromic strings such as:

• Using the standard (simple) method
• Using function in C
• Using String library function of reverse and compare in C
• Using Recursion

1. Using the Standard (simple) Method

Algorithm:

• Initialize 2 variables, l from the start and h from the end.
• now while (h>l), we will check its equivalent character in the string.
• if it’s not equal then it’s not a palindrome 
• else it will traverse half character and the string is a palindrome.

C

// C implementation to check if a given
// string is palindrome or not
#include <stdio.h>
#include <string.h>

int main()
{
    char str[] = { "abbba" };

    // Start from leftmost and
    // rightmost corners of str
    int l = 0;
    int h = strlen(str) - 1;

    // Keep comparing characters
    // while they are same
    while (h > l) {
        if (str[l++] != str[h--]) {
            printf("%s is not a palindrome\n", str);
            return 0;
            // will return from here
        }
    }

    printf("%s is a palindrome\n", str);

    return 0;
}

Output

abbba is a palindrome

Time complexity: O(n) 
Auxiliary Space: O(1)

2. Using function in C

Algorithm of isPalindrome(str) function:

1. Find the length of str. Let length be n. 
2. Initialize low and high indexes as 0 and n-1 respectively. 
3. Do following while low index ‘l’ is smaller than high index ‘h’.
   •  If str[l] is not same as str[h], then return false. 
   • Increment l and decrement h, i.e., do l++ and h–. 
4. If we reach here, it means we didn’t find a mis

C

// C program to check if a string is
// a palindrome or not.
#include <stdio.h>
#include <string.h>

// A function to check if a
// string str is palindrome
void isPalindrome(char str[])
{
    // Start from leftmost and
    // rightmost corners of str
    int l = 0;
    int h = strlen(str) - 1;

    // Keep comparing characters
    // while they are same
    while (h > l)
    {
        if (str[l++] != str[h--])
        {
            printf("%s is not a palindrome\n", str);
            return;
        }
    }
    printf("%s is a palindrome\n", str);
}

// Driver program to test above function
int main()
{
    isPalindrome("abba");
    isPalindrome("abbccbba");
    isPalindrome("geeks");
    return 0;
}

Output

abba is a palindrome
abbccbba is a palindrome
geeks is not a palindrome

Time complexity: O(n) 
Auxiliary Space: O(1)

3. Using String Library Function Compare in C

We can also check a palindromic string with the help of compare function in the C library. Given string is reversed and then it is compared with the help of strcmp() which compares two strings in C. If they are equal then the string will be a palindromic string else not.

C

// C program to check palindromic
// string by reversing
// and comparing
#include <stdio.h>
#include <string.h>

int main()
{

    char str[20];

    printf("Enter String : ");
    scanf("%s", str);

    char s[] = "", t;
    int l = 0, n = 0;

    // length of string
    n = strlen(s);
    for (int i = 0; i <= n / 2; i++) {
        t = s[i];
        s[i] = s[n];
        s[n--] = t;
    }

    // comparing string if they
    // are equal then it's a
    // palindromic string.
    if (!strcmp(str, s) == 0) {
        printf("%s is a palindrome\n", str);
    }
    else {
        printf("%s is not a palindrome\n", str);
    }

    return 0;
}

Output:

Enter String : abba
abba is a palindrome

Time complexity: O(n) 
Auxiliary Space: O(1)

4. Using Recursion

With the help of recursion, we can check the palindromic string in C. In ispalindrome() function lower variable i.e, l is increased and the higher variable i.e, r is decreased and if at any point of time they are not equal then it means the string is not a palindromic string else it will continue till (l>=r) and if all the indices of l and r are equal then we can say that the string is a palindromic string and we will return 1.

C

// C program to check palindromic string by recursion
#include <stdio.h>
#include <string.h>

int isPalindrome(char *str, int l, int r){
     if(NULL == str || l < 0 || r < 0){
         return 0;
     }
     if(l >= r)
         {
             return 1;
         }
     if(str[l] == str[r]){
         return isPalindrome(str, l + 1, r - 1);
     }
     return 0;
 }

int main()
{
    char s[] = {"abbba"} ;
    int in=0;
    int i=0,n=sizeof(s)/sizeof(s[0]);

    if(isPalindrome(s,i,strlen(s) - 1))
         printf("%s is palindrome",s);
    else
        printf("%s is not palindrome",s);

 }

Output

abbba is palindrome

Time complexity: O(n) 
Auxiliary Space: O(1)