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Number of flips to make binary string alternate | Set 1

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 Practice

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)

Implementation:

C++

 `// C++ program to find minimum number of flip to make binary``// string alternate``#include ``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 string starts with 0``    ``//  2) flips when alternate string 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;``}` `// This code is contributed by Sania Kumari Gupta``// (kriSania804)`

C

 `// C program to find minimum number of flip to make binary``// string alternate``#include ``#include ` `// Find minimum between two numbers.``int` `min(``int` `num1, ``int` `num2)``{``    ``return` `(num1 > num2) ? num2 : num1;``}` `//  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(``char` `str[], ``char` `expected)``{``    ``int` `flipCount = 0;``    ``for` `(``int` `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``int` `minFlipToMakeStringAlternate(``char` `str[])``{``    ``//  return minimum of following two``    ``//  1) flips when alternate string starts with 0``    ``//  2) flips when alternate string starts with 1``    ``return` `min(getFlipWithStartingCharcter(str, ``'0'``),``               ``getFlipWithStartingCharcter(str, ``'1'``));``}` `//  Driver code to test above method``int` `main()``{``    ``char` `str[] = ``"0001010111"``;``    ``printf``(``"%d"``,minFlipToMakeStringAlternate(str));``    ``return` `0;``}` `// This code is contributed by Sania Kumari Gupta (kriSania804)`

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 string starts with 0``    ``# 2) flips when alternate string 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

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Javascript

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Output

`2`

Time Complexity: O(N)
Auxiliary Space: O(1)

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