Palindrome Detection in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of palindrome check for a simple string

What is the output of this C code that checks if a string is a palindrome?

DSA C

#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;
        }
        left++;
        right--;
    }
    return 1;
}

int main() {
    char s[] = "radar";
    if (isPalindrome(s)) {
        printf("Palindrome\n");
    } else {
        printf("Not Palindrome\n");
    }
    return 0;
}

A Palindrome

B Not Palindrome

C Compilation Error

D Runtime Error

💡 Hint

Check if the string reads the same forwards and backwards.

The string "radar" is the same when reversed, so the function returns 1 and prints "Palindrome".

❓ Predict Output

intermediate

Output for palindrome check with mixed characters

What will this C program print when checking the string "hello"?

DSA C

#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;
        }
        left++;
        right--;
    }
    return 1;
}

int main() {
    char s[] = "hello";
    if (isPalindrome(s)) {
        printf("Palindrome\n");
    } else {
        printf("Not Palindrome\n");
    }
    return 0;
}

A Palindrome

B Runtime Error

C Compilation Error

D Not Palindrome

💡 Hint

Check if the first and last characters are the same.

The string "hello" is not the same forwards and backwards, so the function returns 0 and prints "Not Palindrome".

🔧 Debug

advanced

Identify the bug in palindrome detection code

What error does this code produce when checking the string "abba"?

DSA C

#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;
        }
        left++;
        right--;
    }
    return 1;
}

int main() {
    char s[] = "abba";
    if (isPalindrome(s)) {
        printf("Palindrome\n");
    } else {
        printf("Not Palindrome\n");
    }
    return 0;
}

A Compilation Error

B Prints "Palindrome"

C Runtime Error (out-of-bounds access)

D Prints "Not Palindrome"

💡 Hint

Check how the right index is initialized.

The right index is set to strlen(str), which is one past the last character index, causing out-of-bounds access and runtime error.

🧠 Conceptual

advanced

Time complexity of palindrome detection

What is the time complexity of checking if a string of length n is a palindrome using the two-pointer approach?

A O(n)

B O(n^2)

C O(log n)

D O(1)

💡 Hint

Consider how many character comparisons are made.

The two-pointer approach compares characters from both ends moving inward, making at most n/2 comparisons, which is O(n).

❓ Predict Output

expert

Output of palindrome check ignoring case and non-alphanumeric characters

What is the output of this C program that checks if a string is a palindrome ignoring case and non-alphanumeric characters?

DSA C

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

int isAlphaNum(char c) {
    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9');
}

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

int main() {
    char s[] = "A man, a plan, a canal: Panama";
    if (isPalindrome(s)) {
        printf("Palindrome\n");
    } else {
        printf("Not Palindrome\n");
    }
    return 0;
}

A Not Palindrome

B Palindrome

C Compilation Error

D Runtime Error

💡 Hint

Ignore spaces, commas, and case differences.

Ignoring non-alphanumeric characters and case, the string reads the same forwards and backwards, so it prints "Palindrome".