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)  

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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++

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#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;
    findSymPairs(arr, 5);
}
  
//This is contributed by Chhavi

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Java

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// 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);
    }
}

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Python3

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# 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

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C#

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// 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

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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.

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



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