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Number of subsequences in a given binary string divisible by 2
  • Difficulty Level : Medium
  • Last Updated : 27 Dec, 2019

Given a binary string str of length N, the task is to find the count of subsequences of str which are divisible by 2. Leading zeros in a sub-sequence is allowed.

Examples:

Input: str = “101”
Output: 2
“0” and “10” are the only subsequences
which are divisible by 2.

Input: str = “10010”
Output: 22

Naive approach: A naive approach will be to generate all possible sub-sequences and check if they are divisible by 2. The time complexity for this will be O(2N * N).



Efficient approach: It can be observed that any binary number is divisible by 2 only if it ends with a 0. Now, the task is to just count the number of subsequences ending with 0. So, for every index i such that str[i] = ‘0’, find the number of subsequences ending at i. This value is equal to 2i (0-based indexing). Thus, the final answer will be equal to the summation of 2i for all i such that str[i] = ‘0’.

Below is the implementation of the above approach:

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the count
// of the required subsequences
int countSubSeq(string str, int len)
{
    // To store the final answer
    int ans = 0;
  
    // Multiplier
    int mul = 1;
  
    // Loop to find the answer
    for (int i = 0; i < len; i++) {
  
        // Condition to update the answer
        if (str[i] == '0')
            ans += mul;
        // updating multiplier
        mul *= 2;
    }
  
    return ans;
}
  
// Driver code
int main()
{
    string str = "10010";
    int len = str.length();
  
    cout << countSubSeq(str, len);
  
    return 0;
}

Java




// Java implementation of the approach
class GFG
{
  
// Function to return the count
// of the required subsequences
static int countSubSeq(String str, int len)
{
    // To store the final answer
    int ans = 0;
  
    // Multiplier
    int mul = 1;
  
    // Loop to find the answer
    for (int i = 0; i < len; i++) 
    {
  
        // Condition to update the answer
        if (str.charAt(i) == '0')
            ans += mul;
              
        // updating multiplier
        mul *= 2;
    }
    return ans;
}
  
// Driver code
public static void main(String[] args)
{
    String str = "10010";
    int len = str.length();
  
    System.out.print(countSubSeq(str, len));
}
}
  
// This code is contributed by 29AjayKumar

Python3




# Python3 implementation of the approach
  
# Function to return the count
# of the required subsequences
def countSubSeq(strr, lenn):
      
    # To store the final answer
    ans = 0
  
    # Multiplier
    mul = 1
  
    # Loop to find the answer
    for i in range(lenn):
  
        # Condition to update the answer
        if (strr[i] == '0'):
            ans += mul
              
        # updating multiplier
        mul *= 2
  
    return ans
  
# Driver code
strr = "10010"
lenn = len(strr)
  
print(countSubSeq(strr, lenn))
  
# This code is contributed by Mohit Kumar

C#




// C# implementation of the approach 
using System;
  
class GFG
{
      
    // Function to return the count 
    // of the required subsequences 
    static int countSubSeq(string str, int len) 
    
        // To store the final answer 
        int ans = 0; 
      
        // Multiplier 
        int mul = 1; 
      
        // Loop to find the answer 
        for (int i = 0; i < len; i++) 
        
      
            // Condition to update the answer 
            if (str[i] == '0'
                ans += mul; 
                  
            // updating multiplier 
            mul *= 2; 
        
        return ans; 
    
      
    // Driver code 
    static public void Main ()
    
        string str = "10010"
        int len = str.Length; 
      
        Console.WriteLine(countSubSeq(str, len)); 
    
}
  
// This code is contributed by AnkitRai01


Output:
22

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