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)  

Naive approach: The idea is to use two nested loops, one for selecting one pair and the second for searching the other symmetric pair in the given array.
The pair are said to be symmetric if arr[i][0] == arr[j][1] and arr[i][1] == arr[j][0] satisfy.

Below is the implementation of the above approach:

C++

// A C++ program to find all symmetric pairs in a given
// array of pairs
#include <bits/stdc++.h>
using namespace std;


void findSymPairs(int arr[][2], int row)
{
    // This loop for selection of one pair
    for (int i = 0; i < row; i++) {

      // This loop for searching of symmetric pair
        for (int j = i + 1; j < row; j++) {

            // Condition of symmetric pair
            if (arr[i][0] == arr[j][1]
                and arr[i][1] == arr[j][0])
            {
                cout << "(" << arr[i][0] << ", "
                     << arr[i][1] << ")" << endl;
            }
        }
    }
}

// Driver method
int main()
{
    int arr[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;
    cout << "Following pairs have symmetric pairs\n";
    findSymPairs(arr, 5);
}

// This is contributed by Arpit Jain

Java

public class GFG
{

  // A Java program to find all symmetric pairs in a given
  // array of pairs
  public static void findSymPairs(int[][] arr, int row)
  {

    // This loop for selection of one pair
    for (int i = 0; i < row; i++) {

      // This loop for searching of symmetric pair
      for (int j = i + 1; j < row; j++) {

        // Condition of symmetric pair
        if (arr[i][0] == arr[j][1]
            && arr[i][1] == arr[j][0]) {
          System.out.print("(");
          System.out.print(arr[i][0]);
          System.out.print(", ");
          System.out.print(arr[i][1]);
          System.out.print(")");
          System.out.print("\n");
        }
      }
    }
  }

  // Driver method
  public static void main(String[] args)
  {
    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;
    System.out.print(
      "Following pairs have symmetric pairs\n");
    findSymPairs(arr, 5);
  }
}
  // This is contributed by Aarti_Rathi

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):

    # This loop for selection of one pair
    for i in range(0, row):

        # This loop for searching of symmetric pair
        for j in range(i + 1, row):

            # Condition of symmetric pair
            if (arr[i][0] == arr[j][1] and arr[i][1] == arr[j][0]):
                print("(",arr[i][0],",",arr[i][1],")")

# 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 Arpit Jain

C#

using System;
public class GFG
{

  // A C# program to find all symmetric pairs in a given
  // array of pairs
  public static void findSymPairs(int[,] arr, int row)
  {

    // This loop for selection of one pair
    for (int i = 0; i < row; i++) {

      // This loop for searching of symmetric pair
      for (int j = i + 1; j < row; j++) {

        // Condition of symmetric pair
        if (arr[i,0] == arr[j,1]
            && arr[i,1] == arr[j,0]) {
          Console.Write("(");
          Console.Write(arr[i,0]);
          Console.Write(", ");
          Console.Write(arr[i,1]);
          Console.Write(")");
          Console.Write("\n");
        }
      }
    }
  }

  // Driver method
  public static void Main(String[] args)
  {
    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;
    Console.Write(
      "Following pairs have symmetric pairs\n");
    findSymPairs(arr, 5);
  }
}
  // This is contributed by Abhijeet Kumar(abhijeet19403)

Output

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

Time Complexity: O(n²) .
Auxiliary Space: O(1)

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 the pair with the second element. The Time Complexity of this solution is O(nLogn).

C++

#include<bits/stdc++.h>
using namespace std;

int binarySearch(vector<pair<int, int>> arr, int i, int j, int n){
    int mid = (i+j)/2;
    if(i>j){
        return -1;
    }
    if(arr[mid].second == n){
        return mid;
    }
    else if(arr[mid].second>n){
        return binarySearch(arr ,i, mid-1, n);
    }
    else if(arr[mid].second<n){
        return binarySearch(arr, mid+1, j, n);
    }
}

void sol(vector<pair<int, int>> arr){
    sort(arr.begin(), arr.end());

    for(int i=0; i<arr.size(); i++){
        int idx = binarySearch(arr, 0, arr.size()-1, arr[i].first);
        if(arr[idx].first == arr[i].second && idx != -1){
            cout<<arr[idx].first<<" "<<arr[idx].second<<endl;
        }
    }
}

int main(){
    vector<pair<int, int>> vec = {{11, 20}, {30, 40}, {5, 10}, {40, 30}, {10, 5}};
    sol(vec);

      return 0;
}

Python3

# A Python3 program to find all symmetric
# pairs in a given array of pairs.

# Print all pairs that have
# a symmetric counterpart
def binarySearch(arr, i, j, n):
    mid = (i+j)//2;
    if(i > j):
        return -1;

    if(arr[mid][1] == n):
        return mid
    elif(arr[mid][1] > n):
        return binarySearch(arr, i, mid - 1, n)
    elif(arr[mid][1] < n):
        return binarySearch(arr, mid + 1, j, n)

def findSymPairs(arr, row):
    arr.sort()
    for i in range(row):
        idx = binarySearch(arr, 0, row-1, arr[i][0])
        if(arr[idx][0] == arr[i][1] and idx != -1):
            print(arr[idx][0]," ",arr[idx][1])

# 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 Arpit Jain

Output

5 10
30 40

An Efficient Solution is to use Hashing. The first element of the pair is used as the 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 the second element as a value.

Below is the implementation of the above approach:

C++

#include<bits/stdc++.h>
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[][2], 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][0];
        int sec   = arr[i][1];

        // 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 << ")" <<endl;

        else  // Else put sec element of this pair in hash
            hM[first] = sec;
    }
}

// Driver method
int main()
{
    int arr[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;
    cout<<"Following pairs have symmetric pairs\n";
    findSymPairs(arr, 5);
}

//This is contributed by Chhavi

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<Integer, Integer> hM = new HashMap<Integer, Integer>();

        // 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

Javascript

<script>
// A Javascript program to find all symmetric pairs in a given array of pairs

    // Print all pairs that have a symmetric counterpart
    function findSymPairs(arr)
    {

        // Creates an empty hashMap hM
        let hM = new Map();

        // Traverse through the given array
        for (let i = 0; i < arr.length; i++)
        {

            // First and second elements of current pair
            let first = arr[i][0];
            let sec   = arr[i][1];

            // Look for second element of this pair in hash
            let 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)
               document.write("(" + sec + ", " + first + ")<br>");

            else  // Else put sec element of this pair in hash
               hM.set(first, sec);
        }
    }

    // Driver method
    let arr = new Array(5);
    for(let i = 0; i < arr.length; i++)
    {
        arr[i] = new Array(2);
        for(let j = 0; j < 2; j++)
        {
            arr[i][j] = 0;
        }
    }


    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 is contributed by unknown2108
</script>

Output

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

Time Complexity: O(n), where n is the size of the given array.
Auxiliary Space: O(n)

This article is contributed by Shivam Agrawal. Please write comments if you find anything incorrect, or if you want to share more information about the topic discussed above.