Recursive function to check if a string is palindrome in C++

We are given a string Str as input. The goal is to find if the input string is a palindrome word or not using a recursive function. Palindrome strings are those strings that when read from front or end form the same word. The strings of length 0 are considered as palindromes. Reversing the palindromes character wise forms, the same string as the original.

Examples of palindromes are:- madam, abcba, malayalam etc

Examples

Input − Str = “malayalam”

Output − Input string is palindrome.

Explanation −

Str[ 0 to 8 ] = malayalam

Reverse Str [ 8 to 0 ] = malayalam

Both strings are same.

Input − Str = “tutorial”

Output − Input string is not a palindrome.

Explanation −

Str[ 0 to 7 ] = tutorial

Reverse Str [ 7 to 0 ] = lairotut

Both strings are different

Approach used in the below program is as follows

In this approach we will check if the string contains a single character, if true then it is palindrome. If not, then recursively traverse the whole string for remaining characters and break the recursion if corresponding characters are different.

Take the input string Str[] and calculate its length.
If length is 0 then set result=1.
Else set result=checkPalindrome(Str, 0, length - 1) where 0 is first index and lenght - 1 is last index
Function checkPalindrome(char str[], int first, int last) returns 0 if any character does not match with its corresponding character in the string.
If indexes first and last are the same then string has one character, then return 1.
If not then check the remaining characters except end characters by first++, last-- and recursive call checkPalindrome(str, first, last).
At the end of all recursions we will get a result.
If it is 1 then the input string is palindrome.
Else input string is not a palindrome.
Print result in main.

Example

#include <bits/stdc++.h>
using namespace std;
int checkPalindrome(char str[], int first, int last){
   if (first < last + 1){
      first++;
      last--;
      return checkPalindrome(str, first, last);
   }

   if (first == last){
      return 1;
   }
   if (str[first] != str[last]){
      return 0;
   }
   return 1;
}
// Driver Code
int main(){
   char Str[] = "madam";
   int result;
   int length = strlen(Str);
   if (length == 0){
      result=1;
   }

   else{
      result=checkPalindrome(Str, 0, length - 1);
   }
   if (result==1){
      cout << "Input string is palindrome.";
   }
   else{
      cout << "Input string is not a palindrome.";
   }
   return 0;
}