Minimum replacements to make adjacent characters unequal in a ternary string

Given a string of ‘0’, ‘1’ and ‘2’. The task is to find the minimum number of replacements such that the adjacent characters are not equal.

Examples:

Input: s = “201220211”
Output: 2
Resultant string after changes is 201210210

Input: s = “0120102”
Output: 0

Approach: The following problem can be solved using greedy method. We can greedily compare every adjacent pair. If the adjacent pairs which is character at ith and i-1th are same, then replace ith th charcater with a character which is not equal to the character at i-1th and i+1th index. In case of the last adjacent pair, just replace it with the character which is not equal to the character at i-1th index.

Below is the implementation of the above approach:

C++

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// C++ program to count the minimal
// replacements such that adjacent characters
// are unequal
#include <bits/stdc++.h>
using namespace std;
  
// Function to count the number of
// minimal replacements
int countMinimalReplacements(string s)
{
    // Find the length of the string
    int n = s.length();
  
    int cnt = 0;
  
    // Iterate in the string
    for (int i = 1; i < n; i++) {
  
        // Check if adjacent is similar
        if (s[i] == s[i - 1]) {
            cnt += 1;
  
            // If not the last pair
            if (i != (n - 1)) {
  
                // Check for character which is
                // not same  in i+1 and i-1
                for (auto it : "012") {
                    if (it != s[i + 1] &&
                        it != s[i - 1]) {
                        s[i] = it;
                        break;
                    }
                }
            }
  
            else // Last pair
            {
                // Check for character which is 
                // not same in i-1 index
                for (auto it : "012") {
                    if (it != s[i - 1]) {
                        s[i] = it;
                        break;
                    }
                }
            }
        }
    }
  
    return cnt;
}
  
// Driver Code
int main()
{
    string s = "201220211";
    cout << countMinimalReplacements(s);
    return 0;
}

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Java

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// Java program to count the minimal
// replacements such that adjacent 
// characters are unequal
class GFG 
{
  
    static final int MAX = 26;
      
    // Function to count the number of
    // minimal replacements
    static int countMinimalReplacements(char[] s) 
    {
        // Find the length of the String
        int n = s.length;
  
        int cnt = 0;
  
        // Iterate in the String
        for (int i = 1; i < n; i++)
        {
  
            // Check if adjacent is similar
            if (s[i] == s[i - 1]) 
            {
                cnt += 1;
  
                // If not the last pair
                if (i != (n - 1)) 
                {
  
                    // Check for character which is
                    // not same in i+1 and i-1
                    for (char it : "012".toCharArray())
                    {
                        if (it != s[i + 1]
                                && it != s[i - 1]) 
                        {
                            s[i] = it;
                            break;
                        }
                    }
                
                else // Last pair
                {
                    // Check for character which is 
                    // not same in i-1 index
                    for (char it : "012".toCharArray())
                    {
                        if (it != s[i - 1])
                        {
                            s[i] = it;
                            break;
                        }
                    }
                }
            }
        }
        return cnt;
    }
  
    // Driver Code
    public static void main(String[] args) 
    {
        String s = "201220211";
        System.out.println(countMinimalReplacements(s.toCharArray()));
    }
}
  
// This code is contributed by 29AjayKumar

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Python3

# Python 3 program to count the minimal
# replacements such that adjacent
# characters are unequal

# Function to count the number of
# minimal replacements
def countMinimalReplacements(s):

# Find the length of the string
n = len(s)

cnt = 0

# Iterate in the string
for i in range(1, n):

# Check if adjacent is similar
if (s[i] == s[i – 1]):
cnt += 1;

# If not the last pair
if (i != (n – 1)):

# Check for character which is
# not same in i+1 and i-1
s = list(s)
for j in “012”:
if (j != s[i + 1] and
j != s[i – 1]):
s[i] = j
break

s = ”.join(s)

# Last pair
else:

# Check for character which is
# not same in i-1 index
s = list(s)

for k in “012”:
if (k != s[i – 1]):
s[i] = k
break
s = ”.join(s)

return cnt

# Driver Code
if __name__ == ‘__main__’:
s = “201220211”
print(countMinimalReplacements(s))

# This code is contributed by
# Surendra_Gangwar

C#

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// C# program to count the minimal 
// replacements such that adjacent 
// characters are unequal 
using System;
  
class GFG 
    static readonly int MAX = 26; 
      
    // Function to count the number of 
    // minimal replacements 
    static int countMinimalReplacements(char[] s) 
    
        // Find the length of the String 
        int n = s.Length; 
  
        int cnt = 0; 
  
        // Iterate in the String 
        for (int i = 1; i < n; i++) 
        
  
            // Check if adjacent is similar 
            if (s[i] == s[i - 1]) 
            
                cnt += 1; 
  
                // If not the last pair 
                if (i != (n - 1)) 
                
  
                    // Check for character which is 
                    // not same in i+1 and i-1 
                    foreach (char it in "012".ToCharArray()) 
                    
                        if (it != s[i + 1] &&
                            it != s[i - 1]) 
                        
                            s[i] = it; 
                            break
                        
                    
                
                else // Last pair 
                
                    // Check for character which is 
                    // not same in i-1 index 
                    foreach (char it in "012".ToCharArray()) 
                    
                        if (it != s[i - 1]) 
                        
                            s[i] = it; 
                            break
                        
                    
                
            
        
        return cnt; 
    
  
    // Driver Code 
    public static void Main(String[] args) 
    
        String s = "201220211"
        Console.WriteLine(countMinimalReplacements(s.ToCharArray())); 
    
  
// This code is contributed by Rajput-Ji

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PHP

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<?php
// PHP program to count the minimal 
// replacements such that adjacent 
// characters are unequal 
  
// Function to count the number of 
// minimal replacements 
function countMinimalReplacements($s
    // Find the length of the string 
    $n = strlen($s); 
  
    $cnt = 0; 
    $str = "012";
      
    // Iterate in the string 
    for ($i = 1; $i < $n; $i++) 
    
  
        // Check if adjacent is similar 
        if ($s[$i] == $s[$i - 1]) 
        
            $cnt += 1; 
  
            // If not the last pair 
            if ($i != ($n - 1)) 
            
  
                // Check for character which is 
                // not same in i+1 and i-1 
                for ($it = 0 ; $it < strlen($str); $it++) 
                
                    if ($str[$it] != $s[$i + 1] && 
                        $str[$it] != $s[$i - 1]) 
                    
                        $s[$i] = $str[$it]; 
                        break
                    
                
            
  
            else // Last pair 
            
                // Check for character which is 
                // not same in i-1 index 
                for ($it = 0 ; $it< strlen($str); $it++) 
                
                    if ($str[$it] != $s[$i - 1]) 
                    
                        $s[$i] = $str[$it]; 
                        break
                    
                
            
        
    
  
    return $cnt
  
// Driver Code 
$s = "201220211"
echo countMinimalReplacements($s); 
  
// This code is contributed by Ryuga
?>

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Output:

2

Time Complexity : O(n)



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