# Given an array of pairs, find all symmetric pairs in it

Two pairs (a, b) and (c, d) are said to be symmetric if c is equal to b and a is equal to d. For example, (10, 20) and (20, 10) are symmetric. Given an array of pairs find all symmetric pairs in it.

It may be assumed that the first elements of all pairs are distinct.

Example:

```Input: arr[] = {{11, 20}, {30, 40}, {5, 10}, {40, 30}, {10, 5}}
Output: Following pairs have symmetric pairs
(30, 40)
(5, 10)
```

We strongly recommend you to minimize your browser and try this yourself first.

A Simple Solution is to go through every pair, and check every other pair for symmetric. This solution requires O(n2) time.

A Better Solution is to use sorting. Sort all pairs by the first element. For every pair, do a binary search for the second element in the given array, i.e., check if the second element of this pair exists as the first element in the array. If found, then compare the first element of pair with the second element. Time Complexity of this solution is O(nLogn).

An Efficient Solution is to use Hashing. The first element of pair is used as key and the second element is used as the value. The idea is to traverse all pairs one by one. For every pair, check if its second element is in the hash table. If yes, then compare the first element with the value of the matched entry of the hash table. If the value and the first element match, then we found symmetric pairs. Else, insert the first element as a key and second element as value.

Following is the implementation of this idea.

## C/C++

 `#include ` `using` `namespace` `std; ` ` `  `// A C++ program to find all symmetric pairs in a given array of pairs ` `// Print all pairs that have a symmetric counterpart ` `void` `findSymPairs(``int` `arr[], ``int` `row) ` `{ ` `    ``// Creates an empty hashMap hM ` `    ``unordered_map<``int``, ``int``> hM; ` ` `  `    ``// Traverse through the given array ` `    ``for` `(``int` `i = 0; i < row; i++) ` `    ``{ ` `        ``// First and second elements of current pair ` `        ``int` `first = arr[i]; ` `        ``int` `sec   = arr[i]; ` ` `  `        ``// If found and value in hash matches with first ` `        ``// element of this pair, we found symmetry ` `        ``if` `(hM.find(sec) != hM.end() && hM[sec] == first) ` `            ``cout << ``"("` `<< sec << ``", "` `<< first << ``")"` `<

## Java

 `// A Java program to find all symmetric pairs in a given array of pairs ` `import` `java.util.HashMap; ` `  `  `class` `SymmetricPairs { ` `  `  `    ``// Print all pairs that have a symmetric counterpart ` `    ``static` `void` `findSymPairs(``int` `arr[][]) ` `    ``{ ` `        ``// Creates an empty hashMap hM ` `        ``HashMap hM = ``new` `HashMap(); ` `  `  `        ``// Traverse through the given array ` `        ``for` `(``int` `i = ``0``; i < arr.length; i++) ` `        ``{ ` `            ``// First and second elements of current pair ` `            ``int` `first = arr[i][``0``]; ` `            ``int` `sec   = arr[i][``1``]; ` `             `  `            ``// Look for second element of this pair in hash ` `            ``Integer val = hM.get(sec); ` `  `  `            ``// If found and value in hash matches with first ` `            ``// element of this pair, we found symmetry ` `            ``if` `(val != ``null` `&& val == first) ` `               ``System.out.println(``"("` `+ sec + ``", "` `+ first + ``")"``); ` `                `  `            ``else`  `// Else put sec element of this pair in hash ` `               ``hM.put(first, sec); ` `        ``} ` `    ``} ` `  `  `    ``// Driver method ` `    ``public` `static` `void` `main(String arg[]) ` `    ``{ ` `        ``int` `arr[][] = ``new` `int``[``5``][``2``]; ` `        ``arr[``0``][``0``] = ``11``; arr[``0``][``1``] = ``20``; ` `        ``arr[``1``][``0``] = ``30``; arr[``1``][``1``] = ``40``; ` `        ``arr[``2``][``0``] = ``5``;  arr[``2``][``1``] = ``10``; ` `        ``arr[``3``][``0``] = ``40``;  arr[``3``][``1``] = ``30``; ` `        ``arr[``4``][``0``] = ``10``;  arr[``4``][``1``] = ``5``; ` `        ``findSymPairs(arr); ` `    ``} ` `} `

## Python3

 `# A Python3 program to find all symmetric  ` `# pairs in a given array of pairs. ` ` `  `# Print all pairs that have ` `# a symmetric counterpart ` `def` `findSymPairs(arr, row): ` ` `  `    ``# Creates an empty hashMap hM ` `    ``hM ``=` `dict``() ` ` `  `    ``# Traverse through the given array ` `    ``for` `i ``in` `range``(row): ` `         `  `        ``# First and second elements  ` `        ``# of current pair ` `        ``first ``=` `arr[i][``0``] ` `        ``sec ``=` `arr[i][``1``] ` ` `  `        ``# If found and value in hash matches with first ` `        ``# element of this pair, we found symmetry ` `        ``if` `(sec ``in` `hM.keys() ``and` `hM[sec] ``=``=` `first): ` `            ``print``(``"("``, sec,``","``, first, ``")"``) ` ` `  `        ``else``: ``# Else put sec element of ` `              ``# this pair in hash ` `            ``hM[first] ``=` `sec ` ` `  `# Driver Code ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``arr ``=` `[[``0` `for` `i ``in` `range``(``2``)]  ` `              ``for` `i ``in` `range``(``5``)] ` `    ``arr[``0``][``0``], arr[``0``][``1``] ``=` `11``, ``20` `    ``arr[``1``][``0``], arr[``1``][``1``] ``=` `30``, ``40` `    ``arr[``2``][``0``], arr[``2``][``1``] ``=` `5``, ``10` `    ``arr[``3``][``0``], arr[``3``][``1``] ``=` `40``, ``30` `    ``arr[``4``][``0``], arr[``4``][``1``] ``=` `10``, ``5` `    ``findSymPairs(arr, ``5``) ` ` `  `# This code is contributed by Mohit Kumar `

## C#

 `// C# program to find all symmetric  ` `// pairs in a given array of pairs  ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `public` `class` `SymmetricPairs  ` `{ ` ` `  `    ``// Print all pairs that have a symmetric counterpart ` `    ``static` `void` `findSymPairs(``int` `[,]arr) ` `    ``{ ` `        ``// Creates an empty hashMap hM ` `        ``Dictionary<``int``,``int``> hM = ``new` `Dictionary<``int``,``int``>(); ` `        ``int` `val = 0; ` `         `  `        ``// Traverse through the given array ` `        ``for` `(``int` `i = 0; i < arr.GetLength(0); i++) ` `        ``{ ` `            ``// First and second elements of current pair ` `            ``int` `first = arr[i, 0]; ` `            ``int` `sec = arr[i, 1]; ` `             `  `            ``// Look for second element of this pair in hash ` `            ``if``(hM.ContainsKey(sec)) ` `            ``val = hM[sec]; ` `             `  ` `  `            ``// If found and value in hash matches with first ` `            ``// element of this pair, we found symmetry ` `            ``if` `(val != 0 && val == first) ` `            ``Console.WriteLine(``"("` `+ sec + ``", "` `+ first + ``")"``); ` `                 `  `            ``else` `// Else put sec element of this pair in hash ` `            ``hM.Add(first, sec); ` `        ``} ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `Main(String []arg) ` `    ``{ ` `        ``int` `[,]arr = ``new` `int``[5, 2]; ` `        ``arr[0, 0] = 11; arr[0, 1] = 20; ` `        ``arr[1, 0] = 30; arr[1, 1] = 40; ` `        ``arr[2, 0] = 5; arr[2, 1] = 10; ` `        ``arr[3, 0] = 40; arr[3, 1] = 30; ` `        ``arr[4, 0] = 10; arr[4, 1] = 5; ` `        ``findSymPairs(arr); ` `    ``} ` `} ` ` `  `// This code has been contributed by 29AjayKumar `

Output:

```Following pairs have symmetric pairs
(30, 40)
(5, 10)```

Time Complexity of this solution is O(n) under the assumption that hash search and insert methods work in O(1) time.

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