C++ Program to Check Whether a Number is Palindrome or Not

Try it on GfG Practice

A palindrome number is a number that remains the same even if its digits are reversed. In this article, we will learn how to check a given number is a palindrome or not in C++.

The easiest way to check if a number is a palindrome is to simply reverse the original number, then check if both numbers are equal. Let's take an example:

#include <iostream>
using namespace std;

int main() {
    int n=1221;
    int t = n;
    int rev = 0;
    
    // Calculate reverse number rev of a given number n
    while (t > 0) {
        int dig = t % 10;
        rev = rev * 10 + dig;
        t /= 10;
    }
    
    // Compare number n with reverse number rev 
    if (n==rev)
        cout << "Palindrome.";
    else
        cout << "Not Palindrome.";
    return 0;
}

Palindrome.

Explanation: In this program, we reverse the number n using while loop and % modulo operator and storing the reverse number into rev variable. Then compare the original number n with reverse number rev. If they are equal print Palindrome otherwise print Not Palindrome.

There are also some other methods to check for a palindrome number in C. They are as follows:

Using Two Pointers After String Conversion

In two-pointer technique, first convert the number into a string using to_string() function and then take two pointers: one pointing at the start of the string and the other at the end of the string. Compare the characters they point to while moving these pointers towards each other. If all the characters match by the time pointers meets, the number is a palindrome, otherwise it is not.

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n = 1221;
    
    // Convert number into string
    string s = to_string(n);
    int l = 0;
    int r = s.size()-1;
    
    // Compare both characters 
    // pointed by l and r pointer
    while (l<r) {
        if (s[l] != s[r]) {
            cout << "Not Palindrome.";
            return 0;
        }
      
        // Move both pointers
        // towards center.
        l++, r--;
    }
    
    // All characters matched
    // means number is palindrome.
    cout << "Palindrome.";
    return 0;
}

Palindrome.

Note: We can also use to check number n is palindrome if generate a reverse string of a generated string, then check both strings are equal to check number is palindrome.

Using Stack

The approach to push all digits of a number into stack storing them in reverse order. Then compare the digits of stack with number's corresponding digits. If any mismatch occurs, the number is not palindrome, otherwise, it is palindrome.

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n = 1231;
    stack<int> st;
    int t = n;
    
    // Insert each digit from the last of
    // number into stack
    while (t > 0) {
        st.push(t % 10);
        t /= 10;
    }
  
    while (!s.empty()) {
      
        // Check stack top element is equal
        // to last digit of a Number
        if (st.top() != n % 10) {
          cout << "Not Palindrome.";
          return 0;
        }
        
        // Remove the top digit of
        // the stack
        s.pop();
      
        // Remmove last digit of n
        n /= 10;
    }
    
    // All character matched means
    // number is palindrome.
    cout << "Palindrome.";
    return 0;
}

Not Palindrome.

Explanation: In this program, pushing all digits of a number into stack. Compare top element of stack s with last digit of a number n and if any mismatch happen means number n is not palindrome otherwise number is palindrome.