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Minimum swaps required to make a binary string alternating

  • Difficulty Level : Medium
  • Last Updated : 23 Sep, 2021

You are given a binary string of even length and equal number of 0’s and 1’s. What is the minimum number of swaps to make the string alternating. A binary string is alternating if no two consecutive elements are equal.

Examples: 

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Input : 000111
Output : 1
Explanation : Swap index 2 and index 5 to get 010101

Input : 1010
Output : 0 

We may either get a 1 at first position or a zero at first position. We consider two cases and find the minimum of two cases. Note that it is given that there are equal of numbers of 1’s and 0’s in the string and string is of even length.
1. Count number of zeroes at odd position and even position of the string. Let their count be odd_0 and even_0 respectively. 
2. Count number of ones at odd position and even position of the string. Let their count be odd_1 and even_1 respectively. 
3. We will always swap a 1 with a 0 (never a 1 with a 1 or a 0 with a 0). So we just check if our alternating string starts with 0 then the number of swaps is min(even_0, odd_1) and if our alternating string starts with 1 then the number of swaps is min(even_1, odd_0). The answer is min of these two.



Below is the implementation of above approach:

C++




// CPP implementation of the approach
#include<bits/stdc++.h>
using namespace std;
 
    // returns the minimum number of swaps
    // of a binary string
    // passed as the argument
    // to make it alternating
    int countMinSwaps(string st)
    {
 
        int min_swaps = 0;
 
        // counts number of zeroes at odd
        // and even positions
        int odd_0 = 0, even_0 = 0;
 
        // counts number of ones at odd
        // and even positions
        int odd_1 = 0, even_1 = 0;
 
        int n = st.length();
        for (int i = 0; i < n; i++) {
            if (i % 2 == 0) {
                if (st[i] == '1')
                    even_1++;
                else
                    even_0++;
            }
            else {
                if (st[i] == '1')
                    odd_1++;
                else
                    odd_0++;
            }
        }
 
        // alternating string starts with 0
        int cnt_swaps_1 = min(even_0, odd_1);
 
        // alternating string starts with 1
        int cnt_swaps_2 = min(even_1, odd_0);
 
        // calculates the minimum number of swaps
        return min(cnt_swaps_1, cnt_swaps_2);
    }
 
    // Driver code
    int main()
    {
        string st = "000111";
        cout<<countMinSwaps(st)<<endl;
 
         return 0;
    }
 
// This code is contributed by Surendra_Gangwar

Java




// Java implementation of the approach
 
class GFG {
 
    // returns the minimum number of swaps
    // of a binary string
    // passed as the argument
    // to make it alternating
    static int countMinSwaps(String st)
    {
 
        int min_swaps = 0;
 
        // counts number of zeroes at odd
        // and even positions
        int odd_0 = 0, even_0 = 0;
 
        // counts number of ones at odd
        // and even positions
        int odd_1 = 0, even_1 = 0;
 
        int n = st.length();
        for (int i = 0; i < n; i++) {
            if (i % 2 == 0) {
                if (st.charAt(i) == '1')
                    even_1++;
                else
                    even_0++;
            }
            else {
                if (st.charAt(i) == '1')
                    odd_1++;
                else
                    odd_0++;
            }
        }
 
        // alternating string starts with 0
        int cnt_swaps_1 = Math.min(even_0, odd_1);
 
        // alternating string starts with 1
        int cnt_swaps_2 = Math.min(even_1, odd_0);
 
        // calculates the minimum number of swaps
        return Math.min(cnt_swaps_1, cnt_swaps_2);
    }
 
    // Driver code
    public static void main(String[] args)
    {
        String st = "000111";
        System.out.println(countMinSwaps(st));
    }
}

Python3




# Python3 implementation of the
# above approach
 
# returns the minimum number of swaps
# of a binary string
# passed as the argument
# to make it alternating
def countMinSwaps(st) :
 
    min_swaps = 0
 
    # counts number of zeroes at odd
    # and even positions
    odd_0, even_0 = 0, 0
 
    # counts number of ones at odd
    # and even positions
    odd_1, even_1 = 0, 0
 
    n = len(st)
 
    for i in range(0, n) :
 
        if i % 2 == 0 :
 
            if st[i] == "1" :
                even_1 += 1
            else :
                even_0 += 1
                 
        else :
            if st[i] == "1" :
                odd_1 += 1
            else :
                odd_0 += 1
 
    # alternating string starts with 0
    cnt_swaps_1 = min(even_0, odd_1)
 
    # alternating string starts with 1
    cnt_swaps_2 = min(even_1, odd_0)
 
    # calculates the minimum number of swaps
    return min(cnt_swaps_1, cnt_swaps_2)
 
# Driver code    
if __name__ == "__main__" :
 
    st = "000111"
 
    # Function call
    print(countMinSwaps(st))
 
# This code is contributed by
# ANKITRAI1

C#




// C# implementation of the approach
using System;
 
public class GFG
{
 
    // returns the minimum number of swaps
    // of a binary string
    // passed as the argument
    // to make it alternating
    public static int countMinSwaps(string st)
    {
 
        int min_swaps = 0;
 
        // counts number of zeroes at odd 
        // and even positions
        int odd_0 = 0, even_0 = 0;
 
        // counts number of ones at odd 
        // and even positions
        int odd_1 = 0, even_1 = 0;
 
        int n = st.Length;
        for (int i = 0; i < n; i++)
        {
            if (i % 2 == 0)
            {
                if (st[i] == '1')
                {
                    even_1++;
                }
                else
                {
                    even_0++;
                }
            }
            else
            {
                if (st[i] == '1')
                {
                    odd_1++;
                }
                else
                {
                    odd_0++;
                }
            }
        }
 
        // alternating string starts with 0
        int cnt_swaps_1 = Math.Min(even_0, odd_1);
 
        // alternating string starts with 1
        int cnt_swaps_2 = Math.Min(even_1, odd_0);
 
        // calculates the minimum number of swaps
        return Math.Min(cnt_swaps_1, cnt_swaps_2);
    }
 
    // Driver code
    public static void Main(string[] args)
    {
        string st = "000111";
        Console.WriteLine(countMinSwaps(st));
    }
}
 
// This code is contributed by Shrikant13

PHP




<?php
// PHP implementation of the approach
// returns the minimum number of swaps
// of a binary string passed as the
// argument to make it alternating
function countMinSwaps($st)
{
    $min_swaps = 0;
 
    // counts number of zeroes at odd
    // and even positions
    $odd_0 = 0;
    $even_0 = 0;
 
    // counts number of ones at odd
    // and even positions
    $odd_1 = 0;
    $even_1 = 0;
 
    $n = strlen($st);
    for ($i = 0; $i < $n; $i++)
    {
        if ($i % 2 == 0)
        {
            if ($st[$i] == '1')
            {
                $even_1++;
            }
            else
            {
                $even_0++;
            }
        }
        else
        {
            if ($st[$i] == '1')
            {
                $odd_1++;
            }
            else
            {
                $odd_0++;
            }
        }
    }
 
    // alternating string starts with 0
    $cnt_swaps_1 = min($even_0, $odd_1);
 
    // alternating string starts with 1
    $cnt_swaps_2 = min($even_1, $odd_0);
 
    // calculates the minimum number of swaps
    return min($cnt_swaps_1, $cnt_swaps_2);
}
 
// Driver code
$st = "000111";
echo (countMinSwaps($st));
 
// This code is contributed by Sachin.
?>

Javascript




<script>
    // Javascript implementation of the approach
     
    // returns the minimum number of swaps
    // of a binary string
    // passed as the argument
    // to make it alternating
    function countMinSwaps(st)
    {
  
        let min_swaps = 0;
  
        // counts number of zeroes at odd
        // and even positions
        let odd_0 = 0, even_0 = 0;
  
        // counts number of ones at odd
        // and even positions
        let odd_1 = 0, even_1 = 0;
  
        let n = st.length;
        for (let i = 0; i < n; i++)
        {
            if (i % 2 == 0)
            {
                if (st[i] == '1')
                {
                    even_1++;
                }
                else
                {
                    even_0++;
                }
            }
            else
            {
                if (st[i] == '1')
                {
                    odd_1++;
                }
                else
                {
                    odd_0++;
                }
            }
        }
  
        // alternating string starts with 0
        let cnt_swaps_1 = Math.min(even_0, odd_1);
  
        // alternating string starts with 1
        let cnt_swaps_2 = Math.min(even_1, odd_0);
  
        // calculates the minimum number of swaps
        return Math.min(cnt_swaps_1, cnt_swaps_2);
    }
     
    let st = "000111";
      document.write(countMinSwaps(st));
     
    // This code is contributed by divyesh072019.
</script>
Output: 
1

 

Approach for odd and even length string :

Let string length be N.

In this approach , we will consider three cases :
1. Answer is impossible when total number of ones > total number of zeroes + 1 or total number of zeroes > total number of ones + 1.
2. String is of even length: 
    We will count number of ones on odd positions ( one_odd ) and number of ones on even positions ( one_even ) then answer is min ( N/2 – one_odd , N/2 – one_even ) 
3. String is of odd length :
    Here we consider two cases :
     i) total number of ones > total number of zeroes (then we have put ones on even positions) so, answer is ( (N + 1) / 2 – number of ones at even positions).
    ii) total number of zeroes > total number of ones (then we have put zeroes on even positions) so, answer is ( (N + 1) / 2 – number of zeroes at even positions ). 

C++




// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
 
// function to count minimum swaps
// required to make binary string
// alternating
int countMinSwaps(string s)
{
    int N = s.size();
 
    // stores total number of ones
    int one = 0;
 
    // stores total number of zeroes
    int zero = 0;
 
    for (int i = 0; i < N; i++) {
        if (s[i] == '1')
            one++;
        else
            zero++;
    }
 
    // checking impossible condition
    if (one > zero + 1 || zero > one + 1)
        return -1;
 
    // odd length string
    if (N % 2) {
        // number of even positions
        int num = (N + 1) / 2;
 
        // stores number of zeroes and
        // ones at even positions
        int one_even = 0, zero_even = 0;
 
        for (int i = 0; i < N; i++) {
            if (i % 2 == 0) {
                if (s[i] == '1')
                    one_even++;
                else
                    zero_even++;
            }
        }
 
        if (one > zero)
            return num - one_even;
 
        else
            return num - zero_even;
    }
 
    // even length string
    else {
        // stores number of ones at odd
        // and even position respectively
        int one_odd = 0, one_even = 0;
 
        for (int i = 0; i < N; i++) {
            if (s[i] == '1') {
                if (i % 2)
                    one_odd++;
 
                else
                    one_even++;
            }
        }
 
        return min(N / 2 - one_odd, N / 2 - one_even);
    }
}
 
// Driver code
int main()
{
    string s = "111000";
 
    cout << countMinSwaps(s);
 
    return 0;
}

Java




// Java implementation of the above approach
import java.util.*;
 
class GFG{
 
// function to count minimum swaps
// required to make binary String
// alternating
static int countMinSwaps(String s)
{
    int N = s.length();
 
    // stores total number of ones
    int one = 0;
 
    // stores total number of zeroes
    int zero = 0;
 
    for (int i = 0; i < N; i++) {
        if (s.charAt(i) == '1')
            one++;
        else
            zero++;
    }
 
    // checking impossible condition
    if (one > zero + 1 || zero > one + 1)
        return -1;
 
    // odd length String
    if (N % 2 == 1)
    {
       
        // number of even positions
        int num = (N + 1) / 2;
 
        // stores number of zeroes and
        // ones at even positions
        int one_even = 0, zero_even = 0;
 
        for (int i = 0; i < N; i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '1')
                    one_even++;
                else
                    zero_even++;
            }
        }
 
        if (one > zero)
            return num - one_even;
 
        else
            return num - zero_even;
    }
 
    // even length String
    else
    {
       
        // stores number of ones at odd
        // and even position respectively
        int one_odd = 0, one_even = 0;
 
        for (int i = 0; i < N; i++) {
            if (s.charAt(i) == '1') {
                if (i % 2 == 1)
                    one_odd++;
 
                else
                    one_even++;
            }
        }
 
        return Math.min(N / 2 - one_odd, N / 2 - one_even);
    }
}
 
// Driver code
public static void main(String[] args)
{
    String s = "111000";
 
    System.out.print(countMinSwaps(s));
}
}
 
// This code is contributed by gauravrajput1

Python3




# Python implementation of the above approach
# function to count minimum swaps
# required to make binary string
# alternating
def countMinSwaps(s):
    N = len(s)
 
    # stores total number of ones
    one = 0
 
    # stores total number of zeroes
    zero = 0
 
    for i in range(N):
        if (s[i] == '1'):
            one += 1
        else:
            zero += 1
     
    # checking impossible condition
    if (one > zero + 1 or zero > one + 1):
        return -1
 
    # odd length string
    if (N % 2):
       
        # number of even positions
        num = (N + 1) / 2
 
        # stores number of zeroes and
        # ones at even positions
        one_even = 0
        zero_even = 0
 
        for i in range(N):
            if (i % 2 == 0):
                if (s[i] == '1'):
                    one_even+=1
                else:
                    zero_even+=1
             
        if (one > zero):
            return num - one_even
 
        else:
            return num - zero_even
     
    # even length string
    else:
        # stores number of ones at odd
        # and even position respectively
        one_odd = 0
        one_even = 0
 
        for i in range(N):
            if (s[i] == '1'):
                if (i % 2):
                    one_odd+=1
 
                else:
                    one_even+=1
             
        return min(N // 2 - one_odd, N // 2 - one_even)
 
# Driver code
s = "111000"
print(countMinSwaps(s))
 
# This code is contributed by shivanisinghss2110

C#




// C# implementation of the above approach
using System;
 
class GFG{
 
// Function to count minimum swaps
// required to make binary String
// alternating
static int countMinSwaps(string s)
{
    int N = s.Length;
 
    // Stores total number of ones
    int one = 0;
 
    // Stores total number of zeroes
    int zero = 0;
 
    for(int i = 0; i < N; i++)
    {
        if (s[i] == '1')
            one++;
        else
            zero++;
    }
 
    // Checking impossible condition
    if (one > zero + 1 || zero > one + 1)
        return -1;
 
    // Odd length String
    if (N % 2 == 1)
    {
       
        // Number of even positions
        int num = (N + 1) / 2;
 
        // Stores number of zeroes and
        // ones at even positions
        int one_even = 0, zero_even = 0;
 
        for(int i = 0; i < N; i++)
        {
            if (i % 2 == 0)
            {
                if (s[i] == '1')
                    one_even++;
                else
                    zero_even++;
            }
        }
        if (one > zero)
            return num - one_even;
        else
            return num - zero_even;
    }
 
    // Even length String
    else
    {
         
        // Stores number of ones at odd
        // and even position respectively
        int one_odd = 0, one_even = 0;
 
        for(int i = 0; i < N; i++)
        {
            if (s[i] == '1')
            {
                if (i % 2 == 1)
                    one_odd++;
                else
                    one_even++;
            }
        }
        return Math.Min(N / 2 - one_odd,
                        N / 2 - one_even);
    }
}
 
// Driver code
public static void Main(String[] args)
{
    string s = "111000";
 
    Console.Write(countMinSwaps(s));
}
}
 
// This code is contributed by shivanisinghss2110

Javascript




<script>
// JavaScript implementation of the above approach
// function to count minimum swaps
// required to make binary String
// alternating
function countMinSwaps(s)
{
    var N = s.length;
 
    // stores total number of ones
    var one = 0;
 
    // stores total number of zeroes
    var zero = 0;
 
    for (var i = 0; i < N; i++) {
        if (s.charAt(i) == '1')
            one++;
        else
            zero++;
    }
 
    // checking impossible condition
    if (one > zero + 1 || zero > one + 1)
        return -1;
 
    // odd length String
    if (N % 2 == 1)
    {
       
        // number of even positions
        var num = (N + 1) / 2;
 
        // stores number of zeroes and
        // ones at even positions
        var one_even = 0, zero_even = 0;
 
        for (var i = 0; i < N; i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '1')
                    one_even++;
                else
                    zero_even++;
            }
        }
 
        if (one > zero)
            return num - one_even;
 
        else
            return num - zero_even;
    }
 
    // even length String
    else
    {
       
        // stores number of ones at odd
        // and even position respectively
        var one_odd = 0, one_even = 0;
 
        for (var i = 0; i < N; i++) {
            if (s.charAt(i) == '1') {
                if (i % 2 == 1)
                    one_odd++;
 
                else
                    one_even++;
            }
        }
 
        return Math.min(N / 2 - one_odd, N / 2 - one_even);
    }
}
 
// Driver code
var s = "111000";
 
document.write(countMinSwaps(s));
 
// This code is contributed by shivanisinghss2110
 
</script>
Output
1

Time Complexity : O(string length)




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