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Count distinct pairs with given sum
• Last Updated : 22 Mar, 2021

Given an array arr[] of size N and an integer K, the task is to find the count of distinct pairs in the array whose sum is equal to K.

Examples:

Input: arr[] = { 5, 6, 5, 7, 7, 8 }, K = 13
Output:
Explanation:
Pairs with sum K( = 13) are { (arr[0], arr[5]), (arr[1], arr[3]), (arr[1], arr[4]) }, i.e. {(5, 8), (6, 7), (6, 7)}.
Therefore, distinct pairs with sum K( = 13) are { (arr[0], arr[5]), (arr[1], arr[3]) }.
Therefore, the required output is 2.

Input: arr[] = { 2, 6, 7, 1, 8, 3 }, K = 10
Output : 2
Explanation:
Distinct pairs with sum K( = 13) are { (arr[0], arr[4]), (arr[2], arr[5]) }.
Therefore, the required output is 2.

Naive Approach: The simplest approach to solve this problem is to use Two Pointer technique. The idea is to sort the array and remove all consecutive duplicate elements from the given array. Finally, count the pairs in the given array whose sum is equal to K. Follow the steps below to solve the problem:

• Initialize a variable, say cntPairs, to store the count of distinct pairs of the array with sum K.
• Sort the array in increasing order.
• Initialize two variables, say i = 0, j = N – 1 as the index of left and right pointers to traverse the array.
• Traverse the array and check for the following conditions:
• Finally, print the value of cntPairs.

Below is the implementation of the above approach:

## C++

 `// C++ program to implement``// the above approach` `#include ``using` `namespace` `std;` `// Function to count distinct pairs``// in array whose sum equal to K``int` `cntDisPairs(``int` `arr[],``                ``int` `N, ``int` `K)``{``    ``// Stores count of distinct pairs``    ``// whose sum equal to K``    ``int` `cntPairs = 0;` `    ``// Sort the array``    ``sort(arr, arr + N);` `    ``// Stores index of``    ``// the left pointer``    ``int` `i = 0;` `    ``// Stores index of``    ``// the right pointer``    ``int` `j = N - 1;` `    ``// Calculate count of distinct``    ``// pairs whose sum equal to K``    ``while` `(i < j) {` `        ``// If sum of current pair``        ``// is equal to K``        ``if` `(arr[i] + arr[j] == K) {` `            ``// Remove consecutive duplicate``            ``// array elements``            ``while` `(i < j && arr[i] == arr[i + 1]) {` `                ``// Update i``                ``i++;``            ``}` `            ``// Remove consecutive duplicate``            ``// array elements``            ``while` `(i < j && arr[j] == arr[j - 1]) {` `                ``// Update j``                ``j--;``            ``}` `            ``// Update cntPairs``            ``cntPairs += 1;` `            ``// Update i``            ``i++;` `            ``// Update j``            ``j--;``        ``}` `        ``// if sum of current pair``        ``// less than K``        ``else` `if` `(arr[i] + arr[j] < K) {` `            ``// Update i``            ``i++;``        ``}``        ``else` `{` `            ``// Update j``            ``j--;``        ``}``    ``}``    ``return` `cntPairs;``}` `// Driver Code``int` `main()``{``    ``int` `arr[] = { 5, 6, 5, 7, 7, 8 };``    ``int` `N = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``int` `K = 13;``    ``cout << cntDisPairs(arr, N, K);``}`

## Java

 `// Java program to implement``// the above approach``import` `java.util.*;` `class` `GFG{` `// Function to count distinct pairs``// in array whose sum equal to K``static` `int` `cntDisPairs(``int` `arr[],``                ``int` `N, ``int` `K)``{``    ``// Stores count of distinct pairs``    ``// whose sum equal to K``    ``int` `cntPairs = ``0``;` `    ``// Sort the array``    ``Arrays.sort(arr);` `    ``// Stores index of``    ``// the left pointer``    ``int` `i = ``0``;` `    ``// Stores index of``    ``// the right pointer``    ``int` `j = N - ``1``;` `    ``// Calculate count of distinct``    ``// pairs whose sum equal to K``    ``while` `(i < j) {` `        ``// If sum of current pair``        ``// is equal to K``        ``if` `(arr[i] + arr[j] == K) {` `            ``// Remove consecutive duplicate``            ``// array elements``            ``while` `(i < j && arr[i] == arr[i + ``1``]) {` `                ``// Update i``                ``i++;``            ``}` `            ``// Remove consecutive duplicate``            ``// array elements``            ``while` `(i < j && arr[j] == arr[j - ``1``]) {` `                ``// Update j``                ``j--;``            ``}` `            ``// Update cntPairs``            ``cntPairs += ``1``;` `            ``// Update i``            ``i++;` `            ``// Update j``            ``j--;``        ``}` `        ``// if sum of current pair``        ``// less than K``        ``else` `if` `(arr[i] + arr[j] < K) {` `            ``// Update i``            ``i++;``        ``}``        ``else` `{` `            ``// Update j``            ``j--;``        ``}``    ``}``    ``return` `cntPairs;``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ``int` `arr[] = { ``5``, ``6``, ``5``, ``7``, ``7``, ``8` `};``    ``int` `N = arr.length;``    ``int` `K = ``13``;``    ``System.out.print(cntDisPairs(arr, N, K));``}` `  ` `// This code is contributed by 29AjayKumar`

## Python3

 `# Python3 program to implement``# the above approach` `# Function to count distinct pairs``# in array whose sum equal to K``def` `cntDisPairs(arr, N, K):``    ` `    ``# Stores count of distinct pairs``    ``# whose sum equal to K``    ``cntPairs ``=` `0` `    ``# Sort the array``    ``arr ``=` `sorted``(arr)` `    ``# Stores index of``    ``# the left pointer``    ``i ``=` `0` `    ``# Stores index of``    ``# the right pointer``    ``j ``=` `N ``-` `1` `    ``# Calculate count of distinct``    ``# pairs whose sum equal to K``    ``while` `(i < j):` `        ``# If sum of current pair``        ``# is equal to K``        ``if` `(arr[i] ``+` `arr[j] ``=``=` `K):` `            ``# Remove consecutive duplicate``            ``# array elements``            ``while` `(i < j ``and` `arr[i] ``=``=` `arr[i ``+` `1``]):` `                ``# Update i``                ``i ``+``=` `1` `            ``# Remove consecutive duplicate``            ``# array elements``            ``while` `(i < j ``and` `arr[j] ``=``=` `arr[j ``-` `1``]):` `                ``# Update j``                ``j ``-``=` `1` `            ``# Update cntPairs``            ``cntPairs ``+``=` `1` `            ``# Update i``            ``i ``+``=` `1` `            ``# Update j``            ``j ``-``=` `1` `        ``# If sum of current pair``        ``# less than K``        ``elif` `(arr[i] ``+` `arr[j] < K):` `            ``# Update i``            ``i ``+``=` `1``        ``else``:` `            ``# Update j``            ``j ``-``=` `1``            ` `    ``return` `cntPairs` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``    ` `    ``arr ``=` `[ ``5``, ``6``, ``5``, ``7``, ``7``, ``8` `]``    ``N ``=` `len``(arr)``    ``K ``=` `13``    ` `    ``print``(cntDisPairs(arr, N, K))` `# This code is contributed by mohit kumar 29`

## C#

 `// C# program to implement``// the above approach``using` `System;``class` `GFG{` `// Function to count distinct pairs``// in array whose sum equal to K``static` `int` `cntDisPairs(``int` `[]arr,``                       ``int` `N, ``int` `K)``{``    ` `    ``// Stores count of distinct pairs``    ``// whose sum equal to K``    ``int` `cntPairs = 0;` `    ``// Sort the array``    ``Array.Sort(arr);` `    ``// Stores index of``    ``// the left pointer``    ``int` `i = 0;` `    ``// Stores index of``    ``// the right pointer``    ``int` `j = N - 1;` `    ``// Calculate count of distinct``    ``// pairs whose sum equal to K``    ``while` `(i < j)``    ``{``        ` `        ``// If sum of current pair``        ``// is equal to K``        ``if` `(arr[i] + arr[j] == K)``        ``{``            ` `            ``// Remove consecutive duplicate``            ``// array elements``            ``while` `(i < j && arr[i] == arr[i + 1])``            ``{``                ` `                ``// Update i``                ``i++;``            ``}` `            ``// Remove consecutive duplicate``            ``// array elements``            ``while` `(i < j && arr[j] == arr[j - 1])``            ``{``                ` `                ``// Update j``                ``j--;``            ``}` `            ``// Update cntPairs``            ``cntPairs += 1;` `            ``// Update i``            ``i++;` `            ``// Update j``            ``j--;``        ``}` `        ``// If sum of current pair``        ``// less than K``        ``else` `if` `(arr[i] + arr[j] < K)``        ``{``            ` `            ``// Update i``            ``i++;``        ``}``        ``else``        ``{``            ` `            ``// Update j``            ``j--;``        ``}``    ``}``    ``return` `cntPairs;``}` `// Driver Code``public` `static` `void` `Main(String[] args)``{``    ``int` `[]arr = { 5, 6, 5, 7, 7, 8 };``    ``int` `N = arr.Length;``    ``int` `K = 13;``    ` `    ``Console.WriteLine(cntDisPairs(arr, N, K));``}``}``  ` `// This code is contributed by jana_sayantan`

## Javascript

 ``
Output:
`2`

Time Complexity: O(N * log(N))
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized using hashing. Follow the steps below to solve the problem:

• Initialize a variable, say cntPairs, to store the count of distinct pairs of the array whose sum is equal to K.
• Initialize a map, say cntFre, to store the frequency of each distinct element of the array.
• Traverse the array and store the frequency of each distinct elements of the array in the map.
• Traverse the map using key value of the map as i and check if the key K – i present in the map or not. If found to be true then increment cntPairs by 1.
• Finally, print the value of cntPairs.

Below is the implementation of the above approach:

## C++

 `// C++ program to implement``// the above approach` `#include ``using` `namespace` `std;` `// Function to count distinct pairs``// in array whose sum equal to K``int` `cntDisPairs(``int` `arr[],``                ``int` `N, ``int` `K)``{``    ``// Stores count of distinct pairs``    ``// whose sum equal to K``    ``int` `cntPairs = 0;` `    ``// Store frequency of each distinct``    ``// element of the array``    ``unordered_map<``int``, ``int``> cntFre;` `    ``for` `(``int` `i = 0; i < N; i++) {` `        ``// Update frequency``        ``// of arr[i]``        ``cntFre[arr[i]]++;``    ``}` `    ``// Traverse the map``    ``for` `(``auto` `it : cntFre) {` `        ``// Stores key value``        ``// of the map``        ``int` `i = it.first;` `        ``// If i is the half of K``        ``if` `(2 * i == K) {` `            ``// If frequency of i``            ``// greater than  1``            ``if` `(cntFre[i] > 1)``                ``cntPairs += 2;``        ``}` `        ``else` `{` `            ``if` `(cntFre[K - i]) {` `                ``// Update cntPairs``                ``cntPairs += 1;``            ``}``        ``}``    ``}` `    ``// Update cntPairs``    ``cntPairs = cntPairs / 2;` `    ``return` `cntPairs;``}` `// Driver Code``int` `main()``{``    ``int` `arr[] = { 5, 6, 5, 7, 7, 8 };``    ``int` `N = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``int` `K = 13;``    ``cout << cntDisPairs(arr, N, K);``}`

## Java

 `// Java program to implement``// the above approach``import` `java.util.*;` `class` `GFG``{` `// Function to count distinct pairs``// in array whose sum equal to K``static` `int` `cntDisPairs(``int` `arr[],``                ``int` `N, ``int` `K)``{``  ` `    ``// Stores count of distinct pairs``    ``// whose sum equal to K``    ``int` `cntPairs = ``0``;` `    ``// Store frequency of each distinct``    ``// element of the array``    ``HashMap cntFre = ``new` `HashMap();` `    ``for` `(``int` `i = ``0``; i < N; i++)``    ``{` `        ``// Update frequency``        ``// of arr[i]``        ``if``(cntFre.containsKey(arr[i]))``            ``cntFre.put(arr[i], cntFre.get(arr[i]) + ``1``);``        ` `        ``else``            ``cntFre.put(arr[i], ``1``);``    ``}` `    ``// Traverse the map``    ``for` `(Map.Entry it : cntFre.entrySet())``    ``{` `        ``// Stores key value``        ``// of the map``        ``int` `i = it.getKey();` `        ``// If i is the half of K``        ``if` `(``2` `* i == K)``        ``{` `            ``// If frequency of i``            ``// greater than  1``            ``if` `(cntFre.get(i) > ``1``)``                ``cntPairs += ``2``;``        ``}` `        ``else``        ``{``            ``if` `(cntFre.containsKey(K - i))``            ``{` `                ``// Update cntPairs``                ``cntPairs += ``1``;``            ``}``        ``}``    ``}` `    ``// Update cntPairs``    ``cntPairs = cntPairs / ``2``;``    ``return` `cntPairs;``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ``int` `arr[] = { ``5``, ``6``, ``5``, ``7``, ``7``, ``8` `};``    ``int` `N = arr.length;``    ``int` `K = ``13``;``    ``System.out.print(cntDisPairs(arr, N, K));``}``}` `// This code  is contributed by shikhasingrajput`

## Python3

 `# Python3 program to implement``# the above approach` `# Function to count distinct pairs``# in array whose sum equal to K``def` `cntDisPairs(arr, N, K):``  ` `    ``# Stores count of distinct pairs``    ``# whose sum equal to K``    ``cntPairs ``=` `0` `    ``# Store frequency of each distinct``    ``# element of the array``    ``cntFre ``=` `{}` `    ``for` `i ``in` `arr:``        ` `        ``# Update frequency``        ``# of arr[i]``        ``if` `i ``in` `cntFre:``            ``cntFre[i] ``+``=` `1``        ``else``:``            ``cntFre[i] ``=` `1` `    ``# Traverse the map``    ``for` `key, value ``in` `cntFre.items():` `        ``# Stores key value``        ``# of the map``        ``i ``=` `key` `        ``# If i is the half of K``        ``if` `(``2` `*` `i ``=``=` `K):``            ` `            ``# If frequency of i``            ``# greater than  1``            ``if` `(cntFre[i] > ``1``):``                ``cntPairs ``+``=` `2``        ``else``:``            ``if` `(cntFre[K ``-` `i]):` `                ``# Update cntPairs``                ``cntPairs ``+``=` `1` `    ``# Update cntPairs``    ``cntPairs ``=` `cntPairs ``/` `2` `    ``return` `cntPairs` `# Driver Code``arr ``=` `[ ``5``, ``6``, ``5``, ``7``, ``7``, ``8` `]``N ``=` `len``(arr)``K ``=` `13``          ` `print``(``int``(cntDisPairs(arr, N, K)))` `# This code is contributed by Dharanendra L V`

## C#

 `// C# program to implement``// the above approach``using` `System;``using` `System.Collections.Generic;``class` `GFG``{` `  ``// Function to count distinct pairs``  ``// in array whose sum equal to K``  ``static` `int` `cntDisPairs(``int` `[]arr,``                         ``int` `N, ``int` `K)``  ``{` `    ``// Stores count of distinct pairs``    ``// whose sum equal to K``    ``int` `cntPairs = 0;` `    ``// Store frequency of each distinct``    ``// element of the array``    ``Dictionary<``int``,``int``> cntFre = ``new` `Dictionary<``int``,``int``>();` `    ``for` `(``int` `i = 0; i < N; i++)``    ``{` `      ``// Update frequency``      ``// of arr[i]``      ``if``(cntFre.ContainsKey(arr[i]))``        ``cntFre[arr[i]] = cntFre[arr[i]] + 1;` `      ``else``        ``cntFre.Add(arr[i], 1);``    ``}` `    ``// Traverse the map``    ``foreach` `(KeyValuePair<``int``,``int``> it ``in` `cntFre)``    ``{` `      ``// Stores key value``      ``// of the map``      ``int` `i = it.Key;` `      ``// If i is the half of K``      ``if` `(2 * i == K)``      ``{` `        ``// If frequency of i``        ``// greater than  1``        ``if` `(cntFre[i] > 1)``          ``cntPairs += 2;``      ``}` `      ``else``      ``{``        ``if` `(cntFre.ContainsKey(K - i))``        ``{` `          ``// Update cntPairs``          ``cntPairs += 1;``        ``}``      ``}``    ``}` `    ``// Update cntPairs``    ``cntPairs = cntPairs / 2;``    ``return` `cntPairs;``  ``}` `  ``// Driver Code``  ``public` `static` `void` `Main(String[] args)``  ``{``    ``int` `[]arr = { 5, 6, 5, 7, 7, 8 };``    ``int` `N = arr.Length;``    ``int` `K = 13;``    ``Console.Write(cntDisPairs(arr, N, K));``  ``}``}` `// This code is contributed by 29AjayKumar`
Output:
`2`

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

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