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

Last Updated : 11 Oct, 2024

A string is said to be palindrome if the reverse of the string is the same as the original string. In this article, we will check whether the given string is palindrome or not in C++.

Examples

Input: str = “ABCDCBA”
Output: “ABCDCBA” is palindrome
Explanation: Reverse of the string str is “ABCDCBA”. So, it is palindrome.

Input: str= “ABCD”
Output: “ABCD” is NOT palindrome
Explanation: Reverse of string str is “DCBA”. So, it is palindrome.

There are various different methods to check whether the string is palindrome or not in C++.

Table of Content

By Comparing with Reversed String

The simplest way to check if a given string is a palindrome is by reversing the string and comparing it with the original string. If they are equal, then the string is palindrome. Otherwise, it is NOT palindrome. We can use the std::reverse() function to reverse the string in C++.

Code Implementation

C++

// C++ program to check whether the given string // is palindrome or not using reverse() function #include <bits/stdc++.h> using namespace std;  void isPalindrome(string str) {      	// Stores the reverse of the string s     string rev = str;      // Reverse the string     reverse(rev.begin(), rev.end());      // If rev is equal to str     if (str == rev)         cout << "\"" << str       	<< "\" is palindrome." << endl;        	// If rev is NOT equal to str     else         cout << "\"" << str       	<< "\" is NOT palindrome." << endl; }  int main() {        	// Checking if strings given strings are palindrome 	isPalindrome("ABCDCBA");   	isPalindrome("ABCD");      return 0; }

Output

Palindrome

Time Complexity: O(n), where n is the length of string.
Auxiliary Space: O(n)

Using Two Pointer Method

In this method, we place the two index pointers, left and right at the starting and ending character of the string respectively. After that, we start matching the character present at left and right position.

If the characters match, we will increment left and decrement right.
This process will repeat till left is less than right. If left will become greater than or equal to right, then the string is palindrome.
If at any point the character does not match, then string is not palindrome.

Code Implementation

C++

// C++ program to check whether the string // is palindrome or not using loop #include <bits/stdc++.h> using namespace std;  void isPalindrome (string str) {      	// Specify the starting and ending indexes     int left = 0, right = str.size() - 1;      	// Flag to terminate the loop if mismatch   	// characters found   	bool flag = true;   	   	// Till the left is less than right     while (left < right) {          // If any character not match, break the         // loop, string is not palindrome         if (str[left] != str[right]) {             flag = false;             break;         }         left++;         right--;     }   	   	if (flag)         cout << "\"" << str       	<< "\" is palindrome." << endl;       	     else         cout << "\"" << str       	<< "\" is NOT palindrome." << endl; }     int main() {      	// Checking if given strings are palindrome   	isPalindrome("ABCDCBA");     isPalindrome("ABCD");                 return 0; }

Output

Palindrome

Time Complexity: O(n), where n is the length of string.
Auxiliary Space: O(1)

Using Recursion

This method basically implements the two-pointer approach using recursion. We pass the left and right index pointer to the recursive function that increment keep increments the left and decrement the right till the mismatched pair of character is found (condition for non-palindrome string) or right becomes less than left (condition for palindrome string).

Code Implementation

C++

// C++ Prgram to check whether the given string is // palindrome or not using recursion #include <iostream> #include <cstring> using namespace std;  // Recursive helper function to check if the string // is a palindrome bool palinHelper(const string& str, int left,                  int right) {      // If the start and end pointers cross each other,   	// it means all characters have matched     if (left >= right)         return true;      // If characters don't match, the string is not   	// a palindrome     if (str[left] != str[right])         return false;      // Recursively check for the rest of the string     return palinHelper(str, left + 1, right - 1); }  // Function to check if the string is a palindrome void isPalindrome(string str) {      // Calling the recursive function to check   	// palindrome string     if (palinHelper(str, 0, str.length() - 1))         cout << "\"" << str << "\" is palindrome."       		<< endl;     else         cout << "\"" << str << "\" is NOT palindrome."       		<< endl; }  int main() {        // Checking if the given strings are palindrome     isPalindrome("ABCDCBA");     isPalindrome("ABCD");      return 0; }

Output

"naman" is palindrome. "vivek" is NOT palindrome.

Java Program to Check Whether a String is a Palindrome

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

By Comparing with Reversed String

Code Implementation

Using Two Pointer Method

Code Implementation

Using Recursion

Code Implementation

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