Example: Longest Palindrome in Array (String Conversion) in C

// Longest Palindrome in Array (String Conversion)
#include <stdio.h>
#include <string.h> // For strlen
#include <stdlib.h> // For malloc, free
#include <limits.h> // For INT_MIN

// Function to check if a string is a palindrome
// Returns 1 if palindrome, 0 otherwise
int isPalindromeString(char *str) {
    int length = strlen(str);
    int start = 0;
    int end = length - 1;

    // Step 1: Use two pointers to compare characters from both ends
    while (start < end) {
        if (str[start] != str[end]) {
            return 0; // Not a palindrome
        }
        start++;
        end--;
    }
    return 1; // It is a palindrome
}

int main() {
    int numbers[] = {121, 23, 4554, 9, 78987, 1001, 5, 987654321, 0, -121};
    int n = sizeof(numbers) / sizeof(numbers[0]);

    int longestPalindrome = INT_MIN;
    int maxDigits = -1;
    int foundPalindrome = 0;

    // Max digits an int can have (for 32-bit int, ~10 digits + sign + null terminator)
    // For long long, it could be up to 20 digits. Let's use 15 for safety.
    char numStr[15]; 

    printf("Array elements: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n\n");

    // Step 1: Iterate through each number in the array
    for (int i = 0; i < n; i++) {
        int currentNum = numbers[i];

        // Step 2: Convert the number to a string
        // sprintf returns the number of characters written, which is the length
        // Handle negative numbers explicitly: negative numbers like -121 would become "-121"
        // which won't be a palindrome as a string.
        if (currentNum < 0) {
            continue; // Skip negative numbers as they are not palindromes by convention
        }
        
        int len = sprintf(numStr, "%d", currentNum);

        // Step 3: Check if the string representation is a palindrome
        if (isPalindromeString(numStr)) {
            foundPalindrome = 1;

            // Step 4: If it's a palindrome, compare its length with the maximum found so far
            if (len > maxDigits) {
                maxDigits = len;
                longestPalindrome = currentNum;
            }
        }
    }

    // Step 5: Print the result
    if (foundPalindrome) {
        printf("The longest palindrome in the array is: %d\n", longestPalindrome);
        printf("It has %d digits.\n", maxDigits);
    } else {
        printf("No palindromes found in the array.\n");
    }

    return 0;
}