Number of flips to make binary string alternate

Given a binary string, that is it contains only 0s and 1s. We need to make this string a sequence of alternate characters by flipping some of the bits, our goal is to minimize the number of bits to be flipped.
Examples :

Input : str = “001”
Output : 1
Minimum number of flips required = 1
We can flip 1st bit from 0 to 1 

Input : str = “0001010111”
Output : 2
Minimum number of flips required = 2
We can flip 2nd bit from 0 to 1 and 9th 
bit from 1 to 0 to make alternate 
string “0101010101”.

Expected time complexity : O(n) where n is length of input string.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

We can solve this problem by considering all possible results, As we are supposed to get alternate string, there are only 2 possibilities, alternate string starting with 0 and alternate string starting with 1. We will try both cases and choose the string which will require minimum number of flips as our final answer.
Trying a case requires O(n) time in which we will loop over all characters of given string, if current character is expected character according to alternation then we will do nothing otherwise we will increase flip count by 1. After trying strings starting with 0 and starting with 1, we will choose the string with minimum flip count.
Total time complexity of solution will be O(n)

C++

// C/C++ program to find minimum number of 
// flip to make binary string alternate 
#include <bits/stdc++.h> 
using namespace std; 
  
//  Utility method to flip a character 
char flip(char ch) 
{ 
    return (ch == '0') ? '1' : '0'; 
} 
  
//  Utility method to get minimum flips when 
//  alternate string starts with expected char 
int getFlipWithStartingCharcter(string str, 
                                char expected) 
{ 
    int flipCount = 0; 
    for (int i = 0; i < str.length(); i++) 
    { 
        //  if current character is not expected, 
        // increase flip count 
        if (str[i] != expected) 
            flipCount++; 
  
        //  flip expected character each time 
        expected = flip(expected); 
    } 
    return flipCount; 
} 
  
// method return minimum flip to make binary 
// string alternate 
int minFlipToMakeStringAlternate(string str) 
{ 
    //  return minimum of following two 
    //  1) flips when alternate strign starts with 0 
    //  2) flips when alternate strign starts with 1 
    return min(getFlipWithStartingCharcter(str, '0'), 
               getFlipWithStartingCharcter(str, '1')); 
} 
  
//  Driver code to test above method 
int main() 
{ 
    string str = "0001010111"; 
    cout << minFlipToMakeStringAlternate(str); 
    return 0; 
} 

Java

// Java program to find minimum number of 
// flip to make binary string alternate 
class GFG  
{ 
    //  Utility method to flip a character 
    public static char flip(char ch) 
    { 
        return (ch == '0') ? '1' : '0'; 
    } 
       
    //  Utility method to get minimum flips when 
    //  alternate string starts with expected char 
    public static int getFlipWithStartingCharcter(String str, 
                                    char expected) 
    { 
        int flipCount = 0; 
        for (int i = 0; i < str.length(); i++) 
        { 
            //  if current character is not expected, 
            // increase flip count 
            if (str.charAt(i) != expected) 
                flipCount++; 
       
            //  flip expected character each time 
            expected = flip(expected); 
        } 
        return flipCount; 
    } 
       
    // method return minimum flip to make binary 
    // string alternate 
    public static int minFlipToMakeStringAlternate(String str) 
    { 
        //  return minimum of following two 
        //  1) flips when alternate string starts with 0 
        //  2) flips when alternate string starts with 1 
        return Math.min(getFlipWithStartingCharcter(str, '0'), 
                   getFlipWithStartingCharcter(str, '1')); 
    } 
       
    //  Driver code to test above method 
    public static void main(String args[]) 
    { 
        String str = "0001010111"; 
        System.out.println(minFlipToMakeStringAlternate(str)); 
    } 
} 
  
// This code is contributed by Sumit Ghosh 

Python 3

# Python 3 program to find minimum number of 
# flip to make binary string alternate 
  
# Utility method to flip a character 
def flip( ch): 
    return '1' if (ch == '0') else '0'
  
# Utility method to get minimum flips when 
# alternate string starts with expected char 
def getFlipWithStartingCharcter(str, expected): 
  
    flipCount = 0
    for i in range(len( str)): 
          
        # if current character is not expected, 
        # increase flip count 
        if (str[i] != expected): 
            flipCount += 1
  
        # flip expected character each time 
        expected = flip(expected) 
    return flipCount 
  
# method return minimum flip to make binary 
# string alternate 
def minFlipToMakeStringAlternate(str): 
  
    # return minimum of following two 
    # 1) flips when alternate strign starts with 0 
    # 2) flips when alternate strign starts with 1 
    return min(getFlipWithStartingCharcter(str, '0'), 
            getFlipWithStartingCharcter(str, '1')) 
  
# Driver code to test above method 
if __name__ == "__main__": 
      
    str = "0001010111"
    print(minFlipToMakeStringAlternate(str)) 

C#

// C# program to find minimum number of 
// flip to make binary string alternate 
using System; 
  
class GFG  
{ 
    // Utility method to 
    // flip a character 
    public static char flip(char ch) 
    { 
        return (ch == '0') ? '1' : '0'; 
    } 
      
    // Utility method to get minimum flips  
    // when alternate string starts with  
    // expected char 
    public static int getFlipWithStartingCharcter(String str, 
                                                char expected) 
    { 
        int flipCount = 0; 
        for (int i = 0; i < str.Length; i++) 
        { 
            // if current character is not  
            // expected, increase flip count 
            if (str[i] != expected) 
                flipCount++; 
      
            // flip expected character each time 
            expected = flip(expected); 
        } 
        return flipCount; 
    } 
      
    // method return minimum flip to  
    // make binary string alternate 
    public static int minFlipToMakeStringAlternate(string str) 
    { 
        // return minimum of following two 
        // 1) flips when alternate string starts with 0 
        // 2) flips when alternate string starts with 1 
        return Math.Min(getFlipWithStartingCharcter(str, '0'), 
                getFlipWithStartingCharcter(str, '1')); 
    } 
      
    // Driver Code 
    public static void Main() 
    { 
        string str = "0001010111"; 
        Console.Write(minFlipToMakeStringAlternate(str)); 
    } 
} 
  
// This code is contributed by nitin mittal. 

PHP

<?php 
// PHP program to find minimum number of 
// flip to make binary string alternate 
  
// Utility method to flip a character 
function flip( $ch) 
{ 
    return ($ch == '0') ? '1' : '0'; 
} 
  
// Utility method to get minimum flips when 
// alternate string starts with expected char 
function getFlipWithStartingCharcter($str, 
                                $expected) 
{ 
      
    $flipCount = 0; 
    for ($i = 0; $i < strlen($str); $i++) 
    { 
          
        // if current character is not expected, 
        // increase flip count 
        if ($str[$i] != $expected) 
            $flipCount++; 
  
        // flip expected character each time 
        $expected = flip($expected); 
    } 
    return $flipCount; 
} 
  
// method return minimum flip to make binary 
// string alternate 
function minFlipToMakeStringAlternate( $str) 
{ 
      
    // return minimum of following two 
    // 1) flips when alternate strign starts with 0 
    // 2) flips when alternate strign starts with 1 
    return min(getFlipWithStartingCharcter($str, '0'), 
               getFlipWithStartingCharcter($str, '1')); 
} 
  
// Driver code to test above method 
$str = "0001010111"; 
echo minFlipToMakeStringAlternate($str); 
  
// This code is contributed by anuj_67. 
?> 

Output :

Minimum number of replacements to make the binary string alternating | Set 2

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My Personal Notes

Number of flips to make binary string alternate | Set 1

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

C++

Java

Python 3

C#

PHP

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