C++ Program to Check if a Given String is Palindrome or Not

A string is called a palindrome if it reads the same from left to right and right to left.

• A palindrome can be checked by comparing the string with its reversed copy.
• The two-pointer approach checks matching characters from both ends without creating a reversed string.

Examples:

Input: ABCDCBA
Output: ABCDCBA is a palindrome.

Input: ABCD
Output: ABCD is not a palindrome.

Approaches to Check if a String is a Palindrome

The following two approaches can be used:

1. Comparing with the Reversed String

In this approach, we create a copy of the string, reverse it using std::reverse(), and compare it with the original string.

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

bool isPalindrome(const string& str)
{
    string rev = str;

    reverse(rev.begin(), rev.end());

    return str == rev;
}

int main()
{
    string str = "ABCDCBA";

    if (isPalindrome(str))
        cout << str << " is a palindrome.\n";
    else
        cout << str << " is not a palindrome.\n";

    return 0;
} 

ABCDCBA is a palindrome.

Explanation: For "ABCDCBA", reversing the string gives:

Original: ABCDCBA
Reversed: ABCDCBA

Since both strings are identical, the given string is a palindrome.

2. Using the Two-Pointer Approach

This approach compares characters from the beginning and end of the string. Two pointers, left and right, move toward the center.

Approach

• Initialize left at the first character and right at the last character.
• Compare the characters at both positions.
• If they differ, the string is not a palindrome.
• If they match, move left forward and right backward.
• Continue until the pointers meet or cross.

#include <iostream>
#include <string>
using namespace std;

bool isPalindrome(const string& str)
{
    int left = 0;
    int right = str.length() - 1;

    while (left < right)
    {
        if (str[left] != str[right])
            return false;

        left++;
        right--;
    }

    return true;
}

int main()
{
    string str = "ABCDCBA";

    if (isPalindrome(str))
        cout << str << " is a palindrome.\n";
    else
        cout << str << " is not a palindrome.\n";

    return 0;
} 

ABCDCBA is a palindrome.

Explanation: The comparisons for "ABCDCBA" are:

A == A
B == B
C == C
D

The pointers reach the middle without finding any mismatch, so the string is a palindrome.

For "ABCD":

A != D

Since the first and last characters are different, the string is not a palindrome.