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Check if the characters in a string form a Palindrome in O(1) extra space
  • Difficulty Level : Medium
  • Last Updated : 28 Dec, 2018

Given a string str. The string may contain lower-case letters, special characters, digits or even white-spaces. The task is to check, whether only the letters present in the string are forming a Palindromic combination or not without using any extra space.

Note: It is not allowed to use extra space to solve this problem. Also, the letters present in the string are in lower-case and the string may contain special characters, digits or even white-spaces along with lowercase letters.

Examples:

Input : str = “m a 343 la y a l am”
Output : YES
The characters in the string form the sequence “malayalam”, which is a palindrome.

Input : str = “malayalam”
Output : YES



Approach:

  • Create two utility functions to get the first and last position of characters present in the string.
  • Start traversing the string, and keep finding the position of first and last characters everytime.
  • If the first and last characters are same for every iteration and the string is traversed completely then print YES otherwise print NO.

Below is the implementation of above approach:

C++

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// CPP program to check if the characters in
// the given string forms a Palindrome in
// O(1) extra space
#include <iostream>
using namespace std;
  
// Utilty function to get the position of
// first character in the string
int firstPos(string str, int start, int end)
{
    int firstChar = -1;
  
    // Get the position of first character
    // in the string
    for (int i = start; i <= end; i++) {
        if (str[i] >= 'a' && str[i] <= 'z') {
            firstChar = i;
            break;
        }
    }
  
    return firstChar;
}
  
// Utilty function to get the position of
// last character in the string
int lastPos(string str, int start, int end)
{
    int lastChar = -1;
  
    // Get the position of last character
    // in the string
    for (int i = start; i >= end; i--) {
        if (str[i] >= 'a' && str[i] <= 'z') {
            lastChar = i;
            break;
        }
    }
  
    return lastChar;
}
  
// Function to check if the characters in
// the given string forms a Palindrome in
// O(1) extra space
bool isPalindrome(string str)
{
    int firstChar = 0, lastChar = str.length() - 1;
    bool ch = true;
  
    for (int i = 0; i < str.length(); i++) {
        firstChar = firstPos(str, firstChar, lastChar);
        lastChar = lastPos(str, lastChar, firstChar);
  
        // break, when all letters are checked
        if (lastChar < 0 || firstChar < 0)
            break;
        if (str[firstChar] == str[lastChar]) {
            firstChar++;
            lastChar--;
            continue;
        }
  
        // if mismatch found, break the loop
        ch = false;
        break;
    }
  
    return (ch);
}
  
// Driver code
int main()
{
    string str = "m     a  343 la y a l am";
    if (isPalindrome(str))
        cout << "YES";
    else
        cout << "NO";
  
    return 0;
}

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Java

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// Java program to check if 
// the characters in the given
// string forms a Palindrome 
// in O(1) extra space
import java.io.*;
  
class GFG 
{
  
// Utilty function to get 
// the position of first
// character in the string
static int firstPos(String str, 
                    int start,
                    int end)
{
    int firstChar = -1;
      
    // Get the position of 
    // first character in
    // the string
    for(int i = start; i <= end; i++)
    {
        if(str.charAt(i) >= 'a'&& 
        str.charAt(i) <= 'z')
        {
            firstChar = i;
            break;
        }
    }
      
    return firstChar;
}
  
// Utilty function to get 
// the position of last 
// character in the string
static int lastPos(String str, 
                int start, 
                int end)
{
    int lastChar = -1;
      
    // Get the position of last 
    // character in the string
    for(int i = start; i >= end; i--)
    {
        if(str.charAt(i) >= 'a'&& 
        str.charAt(i) <= 'z')
        {
            lastChar = i;
            break;
        }
    }
      
    return lastChar;
}
  
// Function to check if the 
// characters in the given 
// string forms a Palindrome 
// in O(1) extra space
static boolean isPalindrome(String str)
{
    int firstChar = 0
        lastChar = str.length() - 1;
    boolean ch = true;
  
    for (int i = 0; i < str.length(); i++) 
    {
        firstChar = firstPos(str, firstChar, 
                                lastChar);
        lastChar = lastPos(str, lastChar,
                                firstChar);
  
        // break, when all 
        // letters are checked
        if (lastChar < 0 || firstChar < 0)
            break;
        if (str.charAt(firstChar) == 
            str.charAt(lastChar)) 
        {
            firstChar++;
            lastChar--;
            continue;
        }
          
        // if mismatch found,
        // break the loop
        ch = false;
        break;
    }
  
        return ch;
  
}
  
// Driver code
public static void main (String[] args) 
{
    String str = "m a 343 la y a l am";
      
    if(isPalindrome(str))
        System.out.print("YES");
    else
    System.out.println("NO");
}
}
  
// This code is contributed 
// by inder_verma.

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Python 3

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# Python 3 program to check if the characters 
# in the given string forms a Palindrome in
# O(1) extra space
  
# Utilty function to get the position of
# first character in the string
def firstPos(str, start, end):
  
    firstChar = -1
  
    # Get the position of first character
    # in the string
    for i in range(start, end + 1):
        if (str[i] >= 'a' and str[i] <= 'z') :
            firstChar = i
            break
  
    return firstChar
  
# Utilty function to get the position of
# last character in the string
def lastPos(str, start, end):
  
    lastChar = -1
  
    # Get the position of last character
    # in the string
    for i in range(start, end - 1, -1) :
        if (str[i] >= 'a' and str[i] <= 'z') :
            lastChar = i
            break
  
    return lastChar
  
# Function to check if the characters in
# the given string forms a Palindrome in
# O(1) extra space
def isPalindrome(str):
  
    firstChar = 0
    lastChar = len(str) - 1
    ch = True
  
    for i in range(len(str)) :
        firstChar = firstPos(str, firstChar, lastChar);
        lastChar = lastPos(str, lastChar, firstChar);
  
        # break, when all letters are checked
        if (lastChar < 0 or firstChar < 0):
            break
        if (str[firstChar] == str[lastChar]):
            firstChar += 1
            lastChar -= 1
            continue
  
        # if mismatch found, break the loop
        ch = False
        break
  
    return (ch)
  
# Driver code
if __name__ == "__main__":
      
    str = "m     a 343 la y a l am"
    if (isPalindrome(str)):
        print("YES")
    else:
        print("NO")
  
# This code is contributed by ita_c

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C#

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// C# program to check if 
// the characters in the given
// string forms a Palindrome 
// in O(1) extra space
using System;
  
class GFG 
{
  
// Utilty function to get 
// the position of first
// character in the string
static int firstPos(string str, 
                    int start,
                    int end)
{
    int firstChar = -1;
      
    // Get the position of 
    // first character in
    // the string
    for(int i = start; i <= end; i++)
    {
        if(str[i] >= 'a'&& 
           str[i] <= 'z')
        {
            firstChar = i;
            break;
        }
    }
      
    return firstChar;
}
  
// Utilty function to get 
// the position of last 
// character in the string
static int lastPos(string str, 
                   int start, 
                   int end)
{
    int lastChar = -1;
      
    // Get the position of last 
    // character in the string
    for(int i = start; i >= end; i--)
    {
        if(str[i] >= 'a'&& 
           str[i] <= 'z')
        {
            lastChar = i;
            break;
        }
    }
      
    return lastChar;
}
  
// Function to check if the 
// characters in the given 
// string forms a Palindrome 
// in O(1) extra space
static bool isPalindrome(string str)
{
    int firstChar = 0, 
        lastChar = str.Length - 1;
    bool ch = true;
  
    for (int i = 0; i < str.Length; i++) 
    {
        firstChar = firstPos(str, firstChar, 
                                  lastChar);
        lastChar = lastPos(str, lastChar,
                                firstChar);
  
        // break, when all 
        // letters are checked
        if (lastChar < 0 || firstChar < 0)
            break;
        if (str[firstChar] == 
            str[lastChar]) 
        {
            firstChar++;
            lastChar--;
            continue;
        }
          
        // if mismatch found,
        // break the loop
        ch = false;
        break;
    }
  
        return ch;
}
  
// Driver code
public static void Main () 
{
    string str = "m a 343 la y a l am";
      
    if(isPalindrome(str))
        Console.WriteLine("YES");
    else
        Console.WriteLine("NO");
}
}
  
// This code is contributed 
// by inder_verma.

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PHP

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<?php
// PHP program to check if the 
// characters in the given 
// string forms a Palindrome 
// in O(1) extra space
  
// Utilty function to get the 
// position of first character
// in the string
function firstPos($str, $start, $end)
{
    $firstChar = -1;
  
    // Get the position of first 
    // character in the string
    for ($i = $start; $i <= $end; $i++) 
    {
        if ($str[$i] >= 'a' and
            $str[$i] <= 'z'
        {
            $firstChar = $i;
            break;
        }
    }
  
    return $firstChar;
}
  
// Utilty function to get the 
// position of last character
// in the string
function lastPos($str, $start, $end)
{
    $lastChar = -1;
  
    // Get the position of last 
    // character in the string
    for ( $i = $start; $i >= $end; $i--) 
    {
        if ($str[$i] >= 'a' and 
            $str[$i] <= 'z'
        {
            $lastChar = $i;
            break;
        }
    }
  
    return $lastChar;
}
  
// Function to check if the 
// characters in the given 
// string forms a Palindrome 
// in O(1) extra space
function isPalindrome($str)
{
    $firstChar = 0; 
    $lastChar = count($str) - 1;
    $ch = true;
  
    for ($i = 0; $i < count($str); $i++) 
    {
        $firstChar = firstPos($str, $firstChar
                                    $lastChar);
        $lastChar = lastPos($str, $lastChar
                                  $firstChar);
  
        // break, when all letters are checked
        if ($lastChar < 0 or $firstChar < 0)
            break;
        if ($str[$firstChar] == $str[$lastChar])
        {
            $firstChar++;
            $lastChar--;
            continue;
        }
  
        // if mismatch found,
        // break the loop
        $ch = false;
        break;
    }
  
    return ($ch);
}
  
// Driver code
$str = "m a 343 la y a l am";
if (isPalindrome($str))
    echo "YES";
else
    echo "NO";
  
// This code is contributed 
// by inder_verma.
?>

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Output:

YES

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