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Count number of common elements between two arrays by using Bitset and Bitwise operation

  • Last Updated : 02 Jul, 2019
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Given two arrays a[] and b[], the task is to find the count of common elements in both the given arrays. Note that both the arrays contain distinct (individually) positive integers.

Examples:

Input: a[] = {1, 2, 3}, b[] = {2, 4, 3}
Output: 2
2 and 3 are common to both the arrays.

Input: a[] = {1, 4, 7, 2, 3}, b[] = {2, 11, 7, 4, 15, 20, 24}
Output: 3

Approach: We will use 3 bitset of same size. First we will traverse first array and set the bit 1 to position a[i] in first bitset.
After that we will traverse second array and set the bit 1 to position b[i] in second bitset.
At last we will find the bitwise AND of both the bitsets and if the ith position of the resultant bitset is 1 then it implies that ith position of first and second bitsets are also 1 and i is the common element in both the arrays.



Below is the implementation of the above approach:

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
#define MAX 100000
bitset<MAX> bit1, bit2, bit3;
  
// Function to return the count of common elements
int count_common(int a[], int n, int b[], int m)
{
  
    // Traverse the first array
    for (int i = 0; i < n; i++) {
  
        // Set 1 at position a[i]
        bit1.set(a[i]);
    }
  
    // Traverse the second array
    for (int i = 0; i < m; i++) {
  
        // Set 1 at position b[i]
        bit2.set(b[i]);
    }
  
    // Bitwise AND of both the bitsets
    bit3 = bit1 & bit2;
  
    // Find the count of 1's
    int count = bit3.count();
  
    return count;
}
  
// Driver code
int main()
{
  
    int a[] = { 1, 4, 7, 2, 3 };
    int b[] = { 2, 11, 7, 4, 15, 20, 24 };
    int n = sizeof(a) / sizeof(a[0]);
    int m = sizeof(b) / sizeof(b[0]);
  
    cout << count_common(a, n, b, m);
  
    return 0;
}

Python3




# Python3 implementation of the approach 
  
MAX = 100000
bit1 , bit2, bit3 = 0, 0, 0
  
# Function to return the count of common elements 
def count_common(a, n, b, m) : 
  
    # Traverse the first array 
    for i in range(n) :
          
        global bit1, bit2, bit3
          
        # Set 1 at (index)position a[i]
        bit1 = bit1 | (1<<a[i])
  
    # Traverse the second array 
    for i in range(m) :
  
        # Set 1 at (index)position b[i] 
        bit2 = bit2 | (1<<b[i])
  
    # Bitwise AND of both the bitsets 
    bit3 = bit1 & bit2; 
  
    # Find the count of 1's 
    count = bin(bit3).count('1'); 
  
    return count; 
  
# Driver code 
if __name__ == "__main__"
  
    a = [ 1, 4, 7, 2, 3 ]; 
    b = [ 2, 11, 7, 4, 15, 20, 24 ]; 
    n = len(a); 
    m = len(b); 
  
    print(count_common(a, n, b, m)); 
  
# This code is contributed by AnkitRai01
Output:
3

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