# 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

Output:

2

This article is contributed by Utkarsh Trivedi. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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