C Exercises: Longest palindromic substring of a string

Last update on August 09 2025 12:47:07 (UTC/GMT +8 hours)

4. Longest Palindromic Substring Variants

Write a C programming to find the longest palindromic substring of a given string.Maximum length of the given string is 1000.

C Code:

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

static inline int min(int x, int y)
 {
     return x < y ? x : y;
 }
static int find_longest_substring(char *s, char output[])
{
    int i, j;
    char s2[3000] = { '\0' };

    s2[0] = '$';
    for (i = 0; s[i] != '\0'; i++) {
        s2[(i<<1)+1] = '#';
        s2[(i<<1)+2] = s[i];
    }
    s2[(i<<1)+1] = '#';
    int len = (i<<1)+2;
    s2[len] = '\0';
    
    int p[3000] = { 0 }; 
    int id = 0;  
    int limit = 0;  
    int max_len = 0;  
    int max_id = 0;  
    for (i = 1; i < len; i++) {
        if (i < limit) {
            p[i] = min(p[2*id-i], limit-i);
        } else {
            p[i] = 1;
        }
        
        while (s2[i+p[i]] == s2[i-p[i]]) {
            p[i]++;
        }
        
        if (i+p[i] > limit) {
            limit = i+p[i];
            id = i;
        }
        if (max_len < p[i]-1) {
            max_len = p[i]-1;
            max_id = i;
        }
    }
    
    for (j = 0, i = max_id - max_len; i <= max_id+max_len; i++) {
        if (s2[i] != '#') {
            output[j++] = s2[i];
        }
    }
    return max_len;
}

static char *longest_Palindrom(char *s)
{
    int i;
    if (s == NULL) {
        return NULL;
    }

    int len = strlen(s);
    if (len <= 1) {
        return s;
    }

    char *palindrome_str = malloc(2000);
    memset(palindrome_str, 0, sizeof(palindrome_str));

    int size = find_longest_substring(s, palindrome_str);
    palindrome_str[size] = '\0';
    return palindrome_str;
}

int main(void)
 {
    char *str1 = "yxypxst";
	printf("\nOriginal String: %s", str1);
	printf("\nLongest palindromic substring of the said string: %s\n", longest_Palindrom(str1));
	return 0;
 }

Sample Output:

Original String: yxypxst
Longest palindromic substring of the said string: yxy

Pictorial Presentation:

C Programming: Find the longest palindromic substring of a given string.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to find the longest palindromic substring using the expand-around-center technique.
Write a C program to return all longest palindromic substrings if more than one exists.
Write a C program to find the longest palindromic substring in a case-insensitive manner.
Write a C program to compute the longest palindromic substring using dynamic programming.

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