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Maximum number of set bits count in a K-size substring of a Binary String

  • Difficulty Level : Medium
  • Last Updated : 18 May, 2021
Geek Week

Given a binary string S of size N and an integer K. The task is to find the maximum number of set bit appears in a substring of size K.

Examples: 

Input: S = “100111010”, K = 3 
Output:
Explanation: 
The substring “111” contains 3 set bits.

Input:S = “0000000”, K = 4 
Output:
Explanation: S doesn’t have any set bits in it, so ans is 0. 
 

Naive Approach:  



  1. Generate all substring of size K.
  2. Find maximum of count of set bits in all substrings.

Time Complexity: O( N2). 
Auxiliary Space: O(1).

Efficient Approach: The problem can be solved using Sliding window technique.  

  1. Take maxcount variable to store maximum count of set bit and Count variable to store count set bit of current window.
  2. Traverse string from 1 to K and calculate the count of set bits and store as maxcount.
  3. Traverse string from K + 1 to length of the string.
  4. At every iteration, decrease count if (K – i)th bit is set. Increase count if ith bit is set. Compare and update maxcount.
  5. After complete array traversal, finally return maxcount.

Below is the implementation of the above approach:  

C++




// C++ program to find the maximum
// set bits in a substring of size K
#include<bits/stdc++.h>
using namespace std;
 
// Function that find Maximum number
// of set bit appears in a substring
// of size K.
int maxSetBitCount(string s, int k)
{
    int maxCount = 0, n = s.length();
    int count = 0;
 
    // Traverse string 1 to k
    for(int i = 0; i < k; i++)
    {
         
       // Increment count if
       // character is set bit
       if (s[i] == '1')
           count++;
    }
    maxCount = count;
 
    // Traverse string k+1
    // to length of string
    for(int i = k; i < n; i++)
    {
        
       // Remove the contribution of the
       // (i - k)th character which is no
       // longer in the window
       if (s[i - k] == '1')
           count--;
        
       // Add the contribution of
       // the current character
       if (s[i] == '1')
           count++;
            
       // Update maxCount at for
       // each window of size k
       maxCount = max(maxCount, count);
    }
     
    // Return maxCount
    return maxCount;
}
 
// Driver code
int main()
{
    string s = "100111010";
    int k = 3;
 
    cout << (maxSetBitCount(s, k));
    return 0;
}
 
// This code is contributed by Rajput-Ji

Java




// Java program to find the maximum
// set bits in a substring of size K
import java.util.*;
 
class GFG {
 
    // Function that find Maximum number
    // of set bit appears in a substring
    // of size K.
    static int maxSetBitCount(String s, int k)
    {
 
        int maxCount = 0, n = s.length();
        int count = 0;
 
        // Traverse string 1 to k
        for (int i = 0; i < k; i++) {
            // Increment count if
            // character is set bit
            if (s.charAt(i) == '1')
                count++;
        }
 
        maxCount = count;
 
        // Traverse string k+1
        // to length of string
        for (int i = k; i < n; i++) {
 
            // remove the contribution of the
            // (i - k)th character which is no
            // longer in the window
            if (s.charAt(i - k) == '1')
                count--;
 
            // add the contribution of
            // the current character
            if (s.charAt(i) == '1')
                count++;
 
            // update maxCount at for
            // each window of size k
            maxCount = Math.max(maxCount, count);
        }
 
        // return maxCount
        return maxCount;
    }
    // Driver Program
    public static void main(String[] args)
    {
        String s = "100111010";
        int k = 3;
 
        System.out.println(maxSetBitCount(s, k));
    }
}

Python3




# Python3 program to find the maximum
# set bits in a substring of size K
 
# Function that find Maximum number
# of set bit appears in a substring
# of size K.
def maxSetBitCount(s, k):
 
    maxCount = 0
    n = len(s)
    count = 0
 
    # Traverse string 1 to k
    for i in range(k):
         
        # Increment count if
        # character is set bit
        if (s[i] == '1'):
            count += 1
 
    maxCount = count
 
    # Traverse string k+1
    # to length of string
    for i in range(k, n):
 
        # Remove the contribution of the
        # (i - k)th character which is no
        # longer in the window
        if (s[i - k] == '1'):
            count -= 1
 
        # Add the contribution of
        # the current character
        if (s[i] == '1'):
            count += 1
 
        # Update maxCount at for
        # each window of size k
        maxCount = max(maxCount, count)
 
    # Return maxCount
    return maxCount
 
 
# Driver code
if __name__ == '__main__':
     
    s = "100111010"
    k = 3
 
    print(maxSetBitCount(s, k))
     
# This code is contributed by mohit kumar 29

C#




// C# program to find the maximum
// set bits in a substring of size K
using System;
class GFG {
 
// Function that find Maximum number
// of set bit appears in a substring
// of size K.
static int maxSetBitCount(string s, int k)
{
 
    int maxCount = 0, n = s.Length;
    int count = 0;
 
    // Traverse string 1 to k
    for (int i = 0; i < k; i++)
    {
        // Increment count if
        // character is set bit
        if (s[i] == '1')
            count++;
    }
 
    maxCount = count;
 
    // Traverse string k+1
    // to length of string
    for (int i = k; i < n; i++)
    {
 
        // remove the contribution of the
        // (i - k)th character which is no
        // longer in the window
        if (s[i - k] == '1')
            count--;
 
        // add the contribution of
        // the current character
        if (s[i] == '1')
            count++;
 
        // update maxCount at for
        // each window of size k
        maxCount = Math.Max(maxCount, count);
    }
 
    // return maxCount
    return maxCount;
}
// Driver Program
public static void Main()
{
    string s = "100111010";
    int k = 3;
 
    Console.Write(maxSetBitCount(s, k));
}
}
 
// This code is contributed by Code_Mech

Javascript




<script>
 
// Javascript program to find the maximum
// set bits in a substring of size K
 
// Function that find Maximum number
// of set bit appears in a substring
// of size K.
function maxSetBitCount(s, k)
{
    var maxCount = 0, n = s.length;
    var count = 0;
 
    // Traverse string 1 to k
    for(var i = 0; i < k; i++)
    {
         
       // Increment count if
       // character is set bit
       if (s[i] == '1')
           count++;
    }
    maxCount = count;
 
    // Traverse string k+1
    // to length of string
    for(var i = k; i < n; i++)
    {
        
       // Remove the contribution of the
       // (i - k)th character which is no
       // longer in the window
       if (s[i - k] == '1')
           count--;
        
       // Add the contribution of
       // the current character
       if (s[i] == '1')
           count++;
            
       // Update maxCount at for
       // each window of size k
       maxCount = Math.max(maxCount, count);
    }
     
    // Return maxCount
    return maxCount;
}
 
// Driver code
var s = "100111010";
var k = 3;
document.write(maxSetBitCount(s, k));
 
// This code is contributed by famously.
</script>
Output: 
3

 

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

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