Count number of common elements between two arrays
Last Updated : 14 Sep, 2024
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 1: 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; } /*package whatever //do not write package name here */ import java.util.*; public class GFG { static int bit1 = 0; static int bit2 = 0; static int bit3 = 0; // Function to return the count of common elements static 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 (index)position a[i] bit1 = bit1 | (1 << a[i]); } // Traverse the second array for (int i = 0; i < m; i++) { // 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 int count = Integer.toBinaryString(bit3).split("1").length - 1; return count; } // Driver code public static void main(String[] args) { int[] a = { 1, 4, 7, 2, 3 }; int[] b = { 2, 11, 7, 4, 15, 20, 24 }; int n = a.length; int m = b.length; System.out.println(count_common(a, n, b, m)); } } // This code is contributed by aadityaburujwale. # 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 // C# implementation of the approach using System; class GFG { static int bit1 = 0; static int bit2 = 0; static int bit3 = 0; // Function to return the count of common elements static 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 (index)position a[i] bit1 = bit1 | (1 << a[i]); } // Traverse the second array for (int i = 0; i < m; i++) { // 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 var count = Convert.ToString(bit3, 2).Split("1").Length - 1; return count; } // Driver code public static void Main(string[] args) { int[] a = { 1, 4, 7, 2, 3 }; int[] b = { 2, 11, 7, 4, 15, 20, 24 }; int n = a.Length; int m = b.Length; Console.WriteLine(count_common(a, n, b, m)); } } // This code is contributed by phasing17 // JavaScript implementation of the approach const MAX = 100000; let bit1 = 0; let bit2 = 0; let bit3 = 0; // Function to return the count of common elements function count_common(a, n, b, m) { // Traverse the first array for (var i = 0; i < n; i++) { // Set 1 at (index)position a[i] bit1 = bit1 | (1<<a[i]); } // Traverse the second array for (var i = 0; i < m; i++) { // 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 var count = bit3.toString(2).split("1").length - 1; return count; } // Driver code var a = [ 1, 4, 7, 2, 3 ]; var b = [ 2, 11, 7, 4, 15, 20, 24 ]; var n = a.length; var m = b.length; console.log(count_common(a, n, b, m)); // This code is contributed by phasing17 Time Complexity: O(n + m)
Auxiliary Space: O(MAX)
Approach 2: We can also use hashmap to store frequencies of each element of both arrays a[] and b[] and sum up the minimum value for each element’s frequency.
Follow given steps to solve the problem:
1. Traverse array a[] and store all frequencies in map freq1.
2. Traverse array b[] and store all frequencies in map freq2.
3. Traverse the map freq1 and sum up the minimum value between x.second and freq2[x.first] in result.
4. Return result as the final answer.
C++14 #include <bits/stdc++.h> using namespace std; int count_common(int *a, int& n, int *b, int& m) { unordered_map<int,int>freq1,freq2; int result=0; for(int i=0;i<n;i++) freq1[a[i]]++; for(int i=0;i<m;i++) freq2[b[i]]++; for(auto& x:freq1) result+=min(x.second,freq2[x.first]); return result; } // driver's code int main() { int a[]={1,2,3}; int n=sizeof(a)/sizeof(a[0]); int b[]={2,4,3}; int m=sizeof(b)/sizeof(b[0]); cout<<count_common(a,n,b,m); return 0; } // this code is contributed by prophet1999 import java.util.*; public class GFG { static int count_common(int[] a, int n, int[] b, int m) { HashMap<Integer, Integer> freq1 = new HashMap<>(); HashMap<Integer, Integer> freq2 = new HashMap<>(); int result = 0; for (int i = 0; i < n; i++) { if (!freq1.containsKey(a[i])) { freq1.put(a[i], 1); } else { freq1.put(a[i], freq1.get(a[i]) + 1); } } for (int i = 0; i < m; i++) { if (!freq2.containsKey(b[i])) { freq2.put(b[i], 1); } else { freq2.put(b[i], freq2.get(b[i]) + 1); } } for (Map.Entry<Integer, Integer> x : freq1.entrySet()) { int p = x.getValue(); int q = 0; if (freq2.containsKey(x.getKey())) { q = freq2.get(x.getKey()); } result += Math.min(p, q); } return result; } // driver's code public static void main(String args[]) { int[] a = { 1, 2, 3 }; int n = a.length; int[] b = { 2, 4, 3 }; int m = b.length; System.out.print(count_common(a, n, b, m)); } } // This code is contributed by Samim Hossain Mondal. def count_common(a, n, b, m): freq1 = {} freq2 = {} result = 0 for element in a: if element in freq1: freq1[element] += 1 else: freq1[element] = 1 for element in b: if element in freq2: freq2[element] += 1 else: freq2[element] = 1 for key, value in freq1.items(): if key in freq2: result += min(value, freq2.get(key)) return result # driver's code a = [1, 2, 3] n = len(a) b = [2, 4, 3] m = len(b) print(count_common(a, n, b, m)) # This code is contributed by Samim Hossain Mondal. using System; using System.Collections.Generic; class GFG { static int count_common(int[] a, int n, int[] b, int m) { Dictionary<int, int> freq1 = new Dictionary<int, int>(); Dictionary<int, int> freq2 = new Dictionary<int, int>(); int result = 0; for (int i = 0; i < n; i++) { if (!freq1.ContainsKey(a[i])) { freq1.Add(a[i], 1); } else { freq1[a[i]]++; } } for (int i = 0; i < m; i++) { if (!freq2.ContainsKey(b[i])) { freq2.Add(b[i], 1); } else { freq2[b[i]]++; } } foreach(KeyValuePair<int, int> x in freq1) { int p = x.Value; int q = 0; if (freq2.ContainsKey(x.Key)) { q = freq2[x.Key]; } result += Math.Min(p, q); } return result; } // driver's code public static void Main() { int[] a = { 1, 2, 3 }; int n = a.Length; int[] b = { 2, 4, 3 }; int m = b.Length; Console.Write(count_common(a, n, b, m)); } } // This code is contributed by Samim Hossain Mondal. function count_common(a, n, b, m) { let freq1 = new Map(); let freq2 = new Map(); let result = 0; for(let i = 0; i < n; i++) if(freq1.has(a[i])) freq1.set(a[i], freq1.get(a[i])+1); else freq1.set(a[i], 1); for(let i = 0; i < m; i++) if(freq2.has(b[i])) freq2.set(b[i], freq2.get(b[i])+1); else freq2.set(b[i], 1); freq1.forEach((value, key) => { if(freq2.has(key)){ result += Math.min(value, freq2.get(key)); } else{ result += Math.min(value, 0); } }); return result; } // driver's code let a = [1,2,3]; let n = a.length; let b = [2,4,3]; let m = b.length; console.log(count_common(a, n, b, m)); // this code is contributed by Samim Hossain Mondal. Time Complexity: O(n+m)
Auxiliary Space: O(n+m)
Approach 3 : We can also use Binary search to check if the element of array a present in the array b or not.
1. Sort the array b in ascending order.
2. Initialize count as 0 , which we store the number of common elements from array a and array b.
3. Iterate each element in array a and use binary search to check if the element exists in array b.
4.If the element exists in array b, increase the count by 1.
5.Return the count .
Below is the implementation of the above approach :
C++ // C++ implementation of the above approach #include <bits/stdc++.h> using namespace std; // Function to check that element x is present in the array bool binarysearch(int arr[], int m, int x) { int l = 0, r = m - 1; while (l <= r) { int mid = (l + r) / 2; // Checking if the middle element is equal to x if (arr[mid] == x) { return true; } else if (arr[mid] < x) { l = mid + 1; } else { r = mid - 1; } } // return true , if element x is present in the array // else false return false; } // Function to count common element int count_common(int a[], int n, int b[], int m) { sort(b, b + m); int count = 0; // Iterate each element of array a for (int i = 0; i < n; i++) { // Checking if the element of array a is present in // array b using the binary search function if (binarysearch(b, m, a[i])) { count++; } } // Return count of common element return count; } // Drive Code int main() { int a[] = { 1, 4, 7, 2, 3 }; int n = sizeof(a) / sizeof(a[0]); int b[] = { 2, 11, 7, 4, 15, 20, 24 }; int m = sizeof(b) / sizeof(b[0]); // Function call cout << "Number of common elements: " << count_common(a, n, b, m) << "\n"; return 0; } // This code is contributed by nikhilsainiofficial546 import java.util.Arrays; class GFG { // Function to check that element x is present in the // array public static boolean binarysearch(int arr[], int m, int x) { int l = 0; int r = m - 1; while (l <= r) { int mid = (l + r) / 2; // Checking if the middle element is equal to x if (arr[mid] == x) { return true; } else if (arr[mid] < x) { l = mid + 1; } else { r = mid - 1; } } // return true , if element x is present in the // array else false return false; } // Function to count common element public static int count_common(int a[], int n, int b[], int m) { Arrays.sort(b); int count = 0; // Iterate each element of array a for (int i = 0; i < n; i++) { // Checking if the element of array a is // present in array b using the binary search // function if (binarysearch(b, m, a[i])) { count++; } } // Return count of common element return count; } // Drive Code public static void main(String[] args) { int a[] = { 1, 4, 7, 2, 3 }; int n = a.length; int b[] = { 2, 11, 7, 4, 15, 20, 24 }; int m = b.length; // Function call System.out.println("Number of common elements: " + count_common(a, n, b, m)); } } // This code is contributed by phasing17. # python3 implementation of the above approach # Function to check that element x is present in the array def binarysearch(arr, m, x): l, r = 0, m - 1 while l <= r: mid = (l + r) // 2 # Checking if the middle element is equal to x if arr[mid] == x: return True elif arr[mid] < x: l = mid + 1 else: r = mid - 1 # return true , if element x is present in the array # else false return False # Function to count common element def count_common(a, n, b, m): b.sort() count = 0 # Iterate each element of array a for i in range(n): # Checking if the element of array a is present in # array b using the binary search function if binarysearch(b, m, a[i]): count += 1 # Return count of common element return count # Drive Code a = [1, 4, 7, 2, 3] n = len(a) b = [2, 11, 7, 4, 15, 20, 24] m = len(b) # Function call print("Number of common elements:", count_common(a, n, b, m)) # This code is contributed by nikhilsainiofficial546 // C# program to count common elements in two arrays using System; using System.Collections; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; class GFG { static void Main(string[] args) { int[] a = new int[] { 1, 4, 7, 2, 3 }; int n = a.Length; int[] b = new int[] { 2, 11, 7, 4, 15, 20, 24 }; int m = b.Length; // Function call Console.WriteLine("Number of common elements: " + count_common(a, n, b, m)); Console.ReadLine(); } // Function to count common element public static int count_common(int[] a, int n, int[] b, int m) { Array.Sort(b); int count = 0; // Iterate each element of array a for (int i = 0; i < n; i++) { // Checking if the element of array a is // present in array b using the binary search // function if (binarysearch(b, m, a[i])) { count++; } } // Return count of common element return count; } // Function to check that element x is present in the // array public static bool binarysearch(int[] arr, int m, int x) { int l = 0; int r = m - 1; while (l <= r) { int mid = (l + r) / 2; // Checking if the middle element is equal to x if (arr[mid] == x) { return true; } else if (arr[mid] < x) { l = mid + 1; } else { r = mid - 1; } } // return true , if element x is present in the // array else false return false; } } // This code is contributed by phasing17. // JavaScript implementation of the above approach // Function to check that element x is present in the array function binarysearch(arr, m, x) { let l = 0; let r = m - 1; while (l <= r) { let mid = Math.floor((l + r) / 2); // Checking if the middle element is equal to x if (arr[mid] === x) { return true; } else if (arr[mid] < x) { l = mid + 1; } else { r = mid - 1; } } // return true , if element x is present in the array // else false return false; } // Function to count common element function count_common(a, n, b, m) { a.sort(function(x, y) { return x - y; }); b.sort(function(x, y) { return x - y; }); let count = 0; // Iterate each element of array a for (let i = 0; i < n; i++) { // Checking if the element of array a is present in // array b using the binary search function if (binarysearch(b, m, a[i])) { count++; } } // Return count of common element return count; } // Drive Code let a = [1, 4, 7, 2, 3]; let n = a.length; let b = [2, 11, 7, 4, 15, 20, 24]; let m = b.length; // Function call console.log("Number of common elements:", count_common(a, n, b, m)); // This code is contributed by phasing17 OutputNumber of common elements: 3
Time Complexity: O(mlogm + nlogm)
Auxiliary Space: O(m)
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