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Minimum number of characters to be replaced to make a given string Palindrome
  • Difficulty Level : Easy
  • Last Updated : 07 Jun, 2019

Given a 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 at 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 ith 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 gets 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),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
// Surendra_Gangwar


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;
?>


Output:

Minimum characters to be replaced =  2
geeeg

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