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# Count pairs from two arrays having sum equal to K

• Difficulty Level : Easy
• Last Updated : 30 Mar, 2021

Given an integer K and two arrays A1 and A2, the task is to return the total number of pairs (one element from A1 and one element from A2) with sum equal to K
Note: Arrays can have duplicate elements. We consider every pair as different, the only constraint is, an element (of any array) can participate only in one pair. For example, A1[] = {3, 3}, A2[] = {4, 4} and K = 7, we consider only two pairs (3, 4) and (3, 4)
Examples:

Input: A1[] = {1, 1, 3, 4, 5, 6, 6}, A2[] = {1, 4, 4, 5, 7}, K = 10
Output:
All possible pairs are {3, 7}, {4, 6}, {5, 5} and {4, 6}

Input: A1[] = {1, 10, 13, 15}, A2[] = {3, 3, 12, 4}, K = 13
Output:

Approach:

• Create a map of the elements of array A1.
• For each element in array A2, check if temp = K – A2[i] exists in map created in previous step.
• If map[temp] > 0 then increment result by 1 and decrement map[temp] by 1.
• Print the total count in the end.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of above approach.` `#include ``using` `namespace` `std;` `// Function to return the count of pairs``// having sum equal to K``int` `countPairs(``int` `A1[], ``int` `A2[]``                  ``, ``int` `n1, ``int` `n2, ``int` `K)``{``    ``// Initialize pairs to 0``    ``int` `res = 0;` `    ``// create map of elements of array A1``    ``unordered_map<``int``, ``int``> m;``    ``for` `(``int` `i = 0; i < n1; ++i)``        ``m[A1[i]]++;` `    ``// count total pairs``    ``for` `(``int` `i = 0; i < n2; ++i) {``        ``int` `temp = K - A2[i];` `        ``if` `(m[temp] != 0) {``            ``res++;` `            ``// Every element can be part``            ``// of at most one pair.``            ``m[temp]--;``        ``}``    ``}` `    ``// return total pairs``    ``return` `res;``}` `// Driver program``int` `main()``{``    ``int` `A1[] = { 1, 1, 3, 4, 5, 6, 6 };``    ``int` `A2[] = { 1, 4, 4, 5, 7 }, K = 10;` `    ``int` `n1 = ``sizeof``(A1) / ``sizeof``(A1);``    ``int` `n2 = ``sizeof``(A2) / ``sizeof``(A2);` `    ``// function call to print required answer``    ``cout << countPairs(A1, A2, n1, n2, K);` `    ``return` `0;``}`

## Java

 `// Java implementation of above approach.``import` `java.util.*;``class` `GfG {` `// Function to return the count of pairs``// having sum equal to K``static` `int` `countPairs(``int` `A1[], ``int` `A2[] , ``int` `n1, ``int` `n2, ``int` `K)``{``    ``// Initialize pairs to 0``    ``int` `res = ``0``;` `    ``// create map of elements of array A1``    ``Map m = ``new` `HashMap ();``    ``for` `(``int` `i = ``0``; i < n1; ++i)``    ``{``        ``if``(m.containsKey(A1[i]))``        ``m.put(A1[i], m.get(A1[i]) + ``1``);``        ``else``        ``m.put(A1[i], ``1``);``    ``}` `    ``// count total pairs``    ``for` `(``int` `i = ``0``; i < n2; ++i) {``        ``int` `temp = K - A2[i];` `        ``if` `(m.containsKey(temp) && m.get(temp) != ``0``) {``            ``res++;` `            ``// Every element can be part``            ``// of at most one pair.``            ``m.put(temp, m.get(A1[i]) - ``1``);``        ``}``    ``}` `    ``// return total pairs``    ``return` `res;``}` `// Driver program``public` `static` `void` `main(String[] args)``{``    ``int` `A1[] = { ``1``, ``1``, ``3``, ``4``, ``5``, ``6``, ``6` `};``    ``int` `A2[] = { ``1``, ``4``, ``4``, ``5``, ``7` `}, K = ``10``;` `    ``int` `n1 = A1.length;``    ``int` `n2 = A2.length;` `    ``// function call to print required answer``    ``System.out.println(countPairs(A1, A2, n1, n2, K));``}``}`

## Python3

 `# Python3 implementation of above approach` `# Function to return the count of``# pairs having sum equal to K``def` `countPairs(A1, A2, n1, n2, K):``    ` `    ``# Initialize pairs to 0``    ``res ``=` `0``    ` `    ``# Create dictionary of elements``    ``# of array A1``    ``m ``=` `dict``()``    ``for` `i ``in` `range``(``0``, n1):``        ``if` `A1[i] ``not` `in` `m.keys():``            ``m[A1[i]] ``=` `1``        ``else``:``            ``m[A1[i]] ``=` `m[A1[i]] ``+` `1``        ` `    ``# count total pairs``    ``for` `i ``in` `range``(``0``, n2):``        ``temp ``=` `K ``-` `A2[i]``        ``if` `temp ``in` `m.keys():``            ``res ``=` `res ``+` `1``            ` `            ``# Every element can be part``            ``# of at most one pair``            ``m[temp] ``=` `m[temp] ``-` `1``    ` `    ``# return total pairs``    ``return` `res` `# Driver Code``A1 ``=` `[``1``, ``1``, ``3``, ``4``, ``5``, ``6` `,``6``]``A2 ``=` `[``1``, ``4``, ``4``, ``5``, ``7``]``K ``=` `10` `n1 ``=` `len``(A1)``n2 ``=` `len``(A2)` `# function call to print required answer``print``(countPairs(A1, A2, n1, n2, K))``        ` `# This code is contributed``# by Shashank_Sharma`

## C#

 `// C# implementation of above approach.``using` `System;``using` `System.Collections.Generic;` `class` `GfG``{` `// Function to return the count of pairs``// having sum equal to K``static` `int` `countPairs(``int` `[]A1, ``int` `[]A2 ,``                        ``int` `n1, ``int` `n2, ``int` `K)``{``    ``// Initialize pairs to 0``    ``int` `res = 0;` `    ``// create map of elements of array A1``    ``Dictionary<``int``,``int``> m = ``new` `Dictionary<``int``,``int``> ();``    ``for` `(``int` `i = 0; i < n1; ++i)``    ``{``        ``int` `a;``        ``if``(m.ContainsKey(A1[i]))``        ``{``            ``a = m[A1[i]] + 1;``            ``m.Remove(A1[i]);``            ``m.Add(A1[i], a);``        ``}``        ``else``        ``m.Add(A1[i], 1);``    ``}` `    ``// count total pairs``    ``for` `(``int` `i = 0; i < n2; ++i)``    ``{``        ``int` `temp = K - A2[i];` `        ``if` `(m.ContainsKey(temp) && m[temp] != 0)``        ``{``            ``res++;` `            ``// Every element can be part``            ``// of at most one pair.``            ``m.Remove(temp);``            ``m.Add(temp, m[A1[i]] - 1);``        ``}``    ``}` `    ``// return total pairs``    ``return` `res;``}` `// Driver program``public` `static` `void` `Main()``{``    ``int` `[]A1 = { 1, 1, 3, 4, 5, 6, 6 };``    ``int` `[]A2 = { 1, 4, 4, 5, 7 };``    ``int` `K = 10;` `    ``int` `n1 = A1.Length;``    ``int` `n2 = A2.Length;` `    ``// function call to print required answer``    ``Console.WriteLine(countPairs(A1, A2, n1, n2, K));``}``}` `/* This code contributed by PrinciRaj1992 */`
Output:
`4`

Time Complexity: O(N+M)

Auxiliary Space: O(N+M)

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