Función recursiva para verificar si una string es palíndromo

Dada una string, escriba una función recursiva que verifique si la string dada es un palíndromo, de lo contrario, no un palíndromo.

Ejemplos: 

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

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

Hemos discutido una función iterativa aquí . 

La idea de una función recursiva es simple: 

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

A continuación se muestra la implementación de la idea anterior: 

// 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;
}
 
// This code is contributed by shivanisinghss2110

// A recursive C program to
// check whether a given number
// is palindrome or not
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
 
// 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))
    printf("Yes");
    else
    printf("No");
 
    return 0;
}

// A recursive JAVA program to
// check whether a given String
// is palindrome or not
import java.io.*;
 
class GFG
{
    // A recursive function that
    // check a str(s..e) is
    // palindrome or not.
    static boolean isPalRec(String 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.charAt(s)) != (str.charAt(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;
    }
 
    static boolean isPalindrome(String str)
    {
        int n = str.length();
 
    // An empty string is
    // considered as palindrome
        if (n == 0)
            return true;
 
        return isPalRec(str, 0, n - 1);
    }
 
    // Driver Code
    public static void main(String args[])
    {
        String str = "geeg";
 
        if (isPalindrome(str))
            System.out.println("Yes");
        else
            System.out.println("No");
    }
}
 
// This code is contributed
// by Nikita Tiwari

# A recursive Python program
# to check whether a given
# number is palindrome or not
 
# A recursive function that
# check a str[s..e] is
# palindrome or not.
def isPalRec(st, s, e) :
     
    # If there is only one character
    if (s == e):
        return True
 
    # If first and last
    # characters do not match
    if (st[s] != st[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(st, s + 1, e - 1);
 
    return True
 
def isPalindrome(st) :
    n = len(st)
     
    # An empty string is
    # considered as palindrome
    if (n == 0) :
        return True
     
    return isPalRec(st, 0, n - 1);
 
 
# Driver Code
st = "geeg"
if (isPalindrome(st)) :
    print "Yes"
else :
    print "No"
     
# This code is contributed
# by Nikita Tiwari.

// A recursive C# program to
// check whether a given number
// is palindrome or not
using System;
 
class GFG
{
 
    // A recursive function that
    // check a str(s..e)
    // is palindrome or not.
    static bool isPalRec(String str,
                         int s,
                         int e)
    {
         
        // If there is only one character
        if (s == e)
            return true;
 
        // If first and last character
        // 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;
    }
 
    static bool isPalindrome(String str)
    {
        int n = str.Length;
 
        // An empty string is considered
        // as palindrome
        if (n == 0)
            return true;
 
        return isPalRec(str, 0, n - 1);
    }
 
    // Driver Code
    public static void Main()
    {
        String str = "geeg";
 
        if (isPalindrome(str))
            Console.Write("Yes");
        else
            Console.Write("No");
    }
}
 
// This code is contributed by Nitin Mittal.

<?php
// A recursive php program to
// check whether a given number
// is palindrome or not
 
 
// A recursive function that
// check a str[s..e] is
// palindrome or not.
function isPalRec($str, $s,$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;
}
 
function isPalindrome($str)
{
$n = strlen($str);
 
// An empty string is
// considered as palindrome
if ($n == 0)
    return true;
 
return isPalRec($str, 0, $n - 1);
}
 
// Driver Code
{
    $str = "geeg";
 
    if (isPalindrome($str))
    echo("Yes");
    else
    echo("No");
 
    return 0;
}
 
// This code is contributed
// by nitin mittal.
?>

<script>
    // A recursive javascript program to
    // check whether a given String
    // is palindrome or not
    // A recursive function that
    // check a str(s..e) is
    // palindrome or not.
    function isPalRec( str , s , e) {
        // If there is only one character
        if (s == e)
            return true;
 
        // If first and last
        // characters do not match
        if ((str.charAt(s)) != (str.charAt(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;
    }
 
    function isPalindrome( str) {
        var n = str.length;
 
        // An empty string is
        // considered as palindrome
        if (n == 0)
            return true;
 
        return isPalRec(str, 0, n - 1);
    }
 
    // Driver Code
     
        var str = "geeg";
 
        if (isPalindrome(str))
            document.write("Yes");
        else
            document.write("No");
 
// This code contributed by gauravrajput1
</script>

Yes

Otro enfoque :

Básicamente, mientras atraviesa, verifique si el índice i-th y ni-1th son iguales o no.

Si no son iguales devuelve falso y si son iguales entonces continúa con las llamadas de recurrencia.

#include <iostream>
using namespace std;
 
bool isPalindrome(string s, int i){
        
    if(i > s.size()/2){
       return true ;
    }
    
    return s[i] == s[s.size()-i-1] && isPalindrome(s, i+1) ;
    
}
     
     
int main()
{
    string str = "geeg" ;
    if (isPalindrome(str, 0))
    cout << "Yes";
    else
    cout << "No";
 
    return 0;
 
}

/*package whatever //do not write package name here */
import java.io.*;
 
class GFG {
  public static boolean isPalindrome(String s, int i){
    if(i > s.length()/2)
    {
      return true ;
    }
 
    return s.charAt(i) == s.charAt(s.length()-i-1) && isPalindrome(s, i+1) ;
 
  }
 
  public static void main (String[] args) {
    String str = "geeg" ;
    if (isPalindrome(str, 0))
    { System.out.println("Yes"); }
    else
    { System.out.println("No"); }
 
  }
}
 
// This code is contributed by akashish.

using System;
 
public class GFG{
 
  public static bool isPalindrome(string s, int i){
 
    if(i > s.Length/2){
      return true ;
    }
 
    return s[i] == s[s.Length-i-1] && isPalindrome(s, i+1) ;
 
  }
 
  public static void Main (){
 
    // Code
    string str = "geeg" ;
    if (isPalindrome(str, 0))
    {
      Console.WriteLine("Yes");
    }
 
    else
    {
      Console.WriteLine("No");
    }
 
 
  }
}
 
// This code is contributed by akashish_.

<script>
function isPalindrome(s,i){
if(i > s.length/2)
        {return true;}
    return s[i] == s[s.length-i-1] && isPalindrome(s, i+1)
} 
     
   let str = "geeg";
let ans = isPalindrome(str, 0);
if (ans == true)
    {
        console.log("Yes");}
else
    {
        console.log("No");}
         
        // This code is contributed by akashish__
</script>
 
 
 
    

Yes

Complejidad temporal: O(n)
Espacio auxiliar: O(n)

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