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Minimum number of characters to be replaced to make a given string Palindrome

  • Difficulty Level : Easy
  • Last Updated : 31 Jul, 2021

Given string str, the task is to find the minimum number of characters to be replaced to make a given string palindrome. Replacing a character means replacing it with any single character in the same position. We are not allowed to remove or add any characters. 

If there are multiple answers, print the lexicographically smallest string.

Examples:  

Input: str = "geeks"
Output: 2
geeks can be converted to geeeg to make it palindrome
by replacing minimum characters.

Input: str = "ameba"
Output: 1
We can get "abeba" or "amema" with only 1 change. 
Among those two, "abeba" is lexicographically smallest. 

Approach: Run a loop from 0 up to (length)/2-1 and check if a character at the i-th index i.e. s[i]!=s[length-i-1], then we will replace the alphabetically larger character with the one which is alphabetically smaller among them and continue the same process until all the elements get traversed.

Below is the implementation of the above approach:  



C++




// C++ Implementation of the above approach
#include<bits/stdc++.h>
using namespace std;
 
// Function to find the minimum number
// character change required
  
void change(string s)
{
 
    // Finding the length of the string
    int n = s.length();
 
    // To store the number of replacement operations
    int cc = 0;
 
    for(int i=0;i<n/2;i++)
    {
 
        // If the characters at location
        // i and n-i-1 are same then
        // no change is required
        if(s[i]== s[n-i-1])
            continue;
 
        // Counting one change operation
        cc+= 1;
 
        // Changing the character with higher
        // ascii value with lower ascii value
        if(s[i]<s[n-i-1])
            s[n-i-1]= s[i] ;
        else
            s[i]= s[n-i-1] ;
    }
    printf("Minimum characters to be replaced = %d\n", (cc)) ;
    cout<<s<<endl;
}
// Driver code
int main()
{
string s = "geeks";
change((s));
return 0;
}
//contributed by Arnab Kundu

Java




// Java Implementation of the above approach
import java.util.*;
 
class GFG
{
 
// Function to find the minimum number
// character change required
static void change(String s)
{
 
    // Finding the length of the string
    int n = s.length();
 
    // To store the number of replacement operations
    int cc = 0;
 
    for(int i = 0; i < n/2; i++)
    {
 
        // If the characters at location
        // i and n-i-1 are same then
        // no change is required
        if(s.charAt(i) == s.charAt(n - i - 1))
            continue;
 
        // Counting one change operation
        cc += 1;
 
        // Changing the character with higher
        // ascii value with lower ascii value
        if(s.charAt(i) < s.charAt(n - i - 1))
            s = s.replace(s.charAt(n - i - 1),s.charAt(i));
        else
            s = s.replace(s.charAt(n-1),s.charAt(n - i - 1));
    }
    System.out.println("Minimum characters to be replaced = "+(cc)) ;
    System.out.println(s);
}
 
// Driver code
public static void main(String args[])
{
    String s = "geeks";
    change((s));
 
}
}
 
// This code is contributed by
// Nikhil Gupta

Python




# Python Implementation of the above approach
 
# Function to find the minimum number
# character change required
import math as ma
def change(s):
 
    # Finding the length of the string
    n = len(s)
 
    # To store the number of replacement operations
    cc = 0
 
    for i in range(n//2):
 
        # If the characters at location
        # i and n-i-1 are same then
        # no change is required
        if(s[i]== s[n-i-1]):
            continue
 
        # Counting one change operation
        cc+= 1
 
        # Changing the character with higher
        # ascii value with lower ascii value
        if(s[i]<s[n-i-1]):
            s[n-i-1]= s[i]
        else:
            s[i]= s[n-i-1]
 
    print("Minimum characters to be replaced = ", str(cc))
    print(*s, sep ="")
     
# Driver code
s = "geeks"
change(list(s))

C#




// C# Implementation of the above approach
using System;
     
class GFG
{
 
// Function to find the minimum number
// character change required
static void change(String s)
{
 
    // Finding the length of the string
    int n = s.Length;
 
    // To store the number of
    //replacement operations
    int cc = 0;
 
    for(int i = 0; i < n / 2; i++)
    {
 
        // If the characters at location
        // i and n-i-1 are same then
        // no change is required
        if(s[i] == s[n - i - 1])
            continue;
 
        // Counting one change operation
        cc += 1;
 
        // Changing the character with higher
        // ascii value with lower ascii value
        if(s[i] < s[n - i - 1])
            s = s.Replace(s[n - i - 1], s[i]);
        else
            s = s.Replace(s[n], s[n - i - 1]);
    }
    Console.WriteLine("Minimum characters " +
                      "to be replaced = " + (cc));
    Console.WriteLine(s);
}
 
// Driver code
public static void Main(String []args)
{
    String s = "geeks";
    change((s));
}
}
 
// This code contributed by Rajput-Ji

PHP




<?php
// PHP Implementation of the above approach
 
// Function to find the minimum number
// character change required
   
function change($s)
{
  
    // Finding the length of the string
    $n = strlen($s);
  
    // To store the number of replacement operations
    $cc = 0;
  
    for($i=0;$i<$n/2;$i++)
    {
  
        // If the characters at location
        // i and n-i-1 are same then
        // no change is required
        if($s[$i]== $s[$n-$i-1])
            continue;
  
        // Counting one change operation
        $cc+= 1;
  
        // Changing the character with higher
        // ascii value with lower ascii value
        if($s[$i]<$s[$n-$i-1])
            $s[$n-$i-1]= $s[$i] ;
        else
            $s[$i]= $s[$n-$i-1] ;
    }
    echo "Minimum characters to be replaced = ". $cc."\n" ;
    echo $s."\n";
}
// Driver code
 
$s = "geeks";
change(($s));
return 0;
?>

Javascript




<script>
 
// Javascript Implementation of the above approach
 
// Function to find the minimum number
// character change required
  
function change(s)
{
 
    // Finding the length of the string
    var n = s.length;
 
    // To store the number of replacement operations
    var cc = 0;
 
    for(var i=0;i<n/2;i++)
    {
 
        // If the characters at location
        // i and n-i-1 are same then
        // no change is required
        if(s[i]== s[n-i-1])
            continue;
 
        // Counting one change operation
        cc+= 1;
 
        // Changing the character with higher
        // ascii value with lower ascii value
        if(s[i]<s[n-i-1])
            s[n-i-1]= s[i] ;
        else
            s[i]= s[n-i-1] ;
    }
    document.write("Minimum characters to be replaced = " + (cc)+"<br>");
    document.write(s.join('') + "<br>");
}
 
// Driver code
var s = "geeks".split('');
change((s));
 
</script>
Output: 
Minimum characters to be replaced =  2
geeeg

 

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