[04 Aug 2020] Recursive function to check if a string is palindrome

Recursive function to check if a string is palindrome

Given a string, write a recursive function that check if the given string is palindrome, else not palindrome.

Examples :

Input : malayalam
Output : Yes
Reverse of malayalam is also
malayalam.

Input : max
Output : No
Reverse of max is not max.

1) If there is only one character in string
   return true.
2) Else compare first and last characters
   and recur for remaining substring.

Below is the implementation of above idea :

C++

// A recursive C++ program to 

// check whether a given number

// is palindrome or not

#include <bits/stdc++.h>

using namespace std;

  

// A recursive function that

// check a str[s..e] is

// palindrome or not.

bool isPalRec(char str[], 

              int s, int e)

{

      

    // If there is only one character

    if (s == e)

    return true;

  

    // If first and last

    // characters do not match

    if (str[s] != str[e])

    return false;

  

    // If there are more than 

    // two characters, check if 

    // middle substring is also 

    // palindrome or not.

    if (s < e + 1)

    return isPalRec(str, s + 1, e - 1);

  

    return true;

}

  

bool isPalindrome(char str[])

{

    int n = strlen(str);

      

    // An empty string is 

    // considered as palindrome

    if (n == 0)

        return true;

      

    return isPalRec(str, 0, n - 1);

}

  

// Driver Code

int main()

{

    char str[] = "geeg";

  

    if (isPalindrome(str))

    cout << "Yes";

    else

    cout << "No";

  

    return 0;

}

Check whether the given string is Palindrome using Stack

Given a string str, the task is to find whether the given string is a palindrome or not using stack.

Examples:

Input: str = “geeksforgeeks”
Output: No

Input: str = “madam”
Output: Yes

// C implementation of the approach #include <malloc.h> #include <stdio.h> #include <stdlib.h> #include <string.h>    char* stack; int top = -1;    // push function void push(char ele) {     stack[++top] = ele; }    // pop function char pop() {     return stack[top--]; }    // Function that returns 1 // if str is a palindrome int isPalindrome(char str[]) {     int length = strlen(str);        // Allocating the memory for the stack     stack = (char*)malloc(length * sizeof(char));        // Finding the mid     int i, mid = length / 2;        for (i = 0; i < mid; i++) {         push(str[i]);     }        // Checking if the length of the string     // is odd, if odd then neglect the     // middle character     if (length % 2 != 0) {         i++;     }        // While not the end of the string     while (str[i] != '\0') {         char ele = pop();            // If the characters differ then the         // given string is not a palindrome         if (ele != str[i])             return 0;         i++;     }        return 1; }    // Driver code int main() {     char str[] = "madam";        if (isPalindrome(str)) {         printf("Yes");     }     else {         printf("No");     }        return 0; }

Output:

Yes