Check if a number is Palindrome

2.9

Given an integer, write a function that returns true if the given number is palindrome, else false. For example, 12321 is palindrome, but 1451 is not palindrome.

check-if-a-number-is-palindrome

Let the given number be num. A simple method for this problem is to first reverse digits of num, then compare the reverse of num with num. If both are same, then return true, else false.

Following is an interesting method inspired from method#2 of this post. The idea is to create a copy of num and recursively pass the copy by reference, and pass num by value. In the recursive calls, divide num by 10 while moving down the recursion tree. While moving up the recursion tree, divide the copy by 10. When they meet in a function for which all child calls are over, the last digit of num will be ith digit from the beginning and the last digit of copy will be ith digit from the end.

// A recursive C++ program to check whether a given number is
// palindrome or not
#include <stdio.h>

// A function that reurns true only if num contains one digit
int oneDigit(int num)
{
    // comparison operation is faster than division operation.
    // So using following instead of "return num / 10 == 0;"
    return (num >= 0 && num < 10);
}

// A recursive function to find out whether num is palindrome
// or not. Initially, dupNum contains address of a copy of num.
bool isPalUtil(int num, int* dupNum)
{
    // Base case (needed for recursion termination): This statement
    // mainly compares the first digit with the last digit
    if (oneDigit(num))
        return (num == (*dupNum) % 10);

    // This is the key line in this method. Note that all recursive
    // calls have a separate copy of num, but they all share same copy
    // of *dupNum. We divide num while moving up the recursion tree
    if (!isPalUtil(num/10, dupNum))
        return false;

    // The following statements are executed when we move up the
    // recursion call tree
    *dupNum /= 10;

    // At this point, if num%10 contains i'th digit from beiginning,
    // then (*dupNum)%10 contains i'th digit from end
    return (num % 10 == (*dupNum) % 10);
}

// The main function that uses recursive function isPalUtil() to
// find out whether num is palindrome or not
int isPal(int num)
{
    // If num is negative, make it positive
    if (num < 0)
       num = -num;

    // Create a separate copy of num, so that modifications made
    // to address dupNum don't change the input number.
    int *dupNum = new int(num); // *dupNum = num

    return isPalUtil(num, dupNum);
}

// Driver program to test above functions
int main()
{
    int n = 12321;
    isPal(n)? printf("Yesn"): printf("Non");

    n = 12;
    isPal(n)? printf("Yesn"): printf("Non");

    n = 88;
    isPal(n)? printf("Yesn"): printf("Non");

    n = 8999;
    isPal(n)? printf("Yesn"): printf("Non");
    return 0;
}

Output:

Yes
No
Yes
No

To check a number is palindrome or not without using any extra space

This article is compiled by Aashish Barnwal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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