Number of flips to make binary string alternate | Set 1
Last Updated :
23 Jul, 2025
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.
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 <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 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 program to find minimum number of flip to make binary
// string alternate
#include <stdio.h>
#include <string.h>
// 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 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 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# 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 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( $str)
{
// return minimum of following two
// 1) flips when alternate string starts with 0
// 2) flips when alternate string starts with 1
return min(getFlipWithStartingCharcter($str, '0'),
getFlipWithStartingCharcter($str, '1'));
}
// Driver code to test above method
$str = "0001010111";
echo minFlipToMakeStringAlternate($str);
// This code is contributed by anuj_67.
?>
<script>
// Javascript 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)
{
let flipCount = 0;
for (let 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
function minFlipToMakeStringAlternate(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
let str = "0001010111";
document.write(minFlipToMakeStringAlternate(str));
// This code is contributed by avanitrachhadiya2155
</script>
Time Complexity: O(N)
Auxiliary Space: O(1)
Minimum number of replacements to make the binary string alternating | Set 2
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