Number of pairs of Array where the max and min of pair is same as their indices

Given an array A[] of N  integers, the task is to calculate the total number of pairs of indices (i, j) satisfying the following conditions –

• 1 ? i < j ? N
• minimum(A[i], A[j]) = i
• maximum(A[i], A[j]) = j

Note: 1-based indexing is considered.

Examples:

Input: N = 4, A[] = {1, 3, 2, 4}
Output: 2
Explanation: First pair of indices is (1, 4),  
As minimum(A[1], A[4]) = minimum(1, 4) = 1 and 
maximum(A[1], A[4]) = maximum(1, 4) = 4.
Similarly, second pair is (3, 2).

Input: N = 10, A[] = {5, 8, 2, 2, 1, 6, 7, 2, 9, 10}
Output: 8

Approach: The problem can be solved based on the following idea: 

The conditions given in the problem would be satisfied, if one of these two conditions holds :

• 1st Type: A[i] = i and A[j] = j
• 2nd Type: A[i] = j and A[j] = i

Say there are K such indices where A[i] = i. So, number of pairs satisfying the first condition is K * (K – 1) / 2.
The number of pairs satisfying the second condition can be simply counted by traversing through the array. 
i.e., checking if A[i] ? i and A[A[i]] = i.

Follow the steps mentioned below to implement the idea: 

• Traverse through the array and find the position where the value and the position are same (say K).
• Find the pairs of the first type mentioned above using the provided formula.
• Now traverse again using and find the second type of pair following the mentioned method.

Below is the implementation of this approach:

C++

// C++ code for the above approach

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

// Function to count Number of pairs in
// array satisfying the given conditions
int countPairs(int N, int A[])
{
    // Variable to store the number of indices such
    // that their value is equal to their position
    int count = 0;

    // Variable to store the total number of pairs
    // following the given condition
    int answer = 0;

    for (int i = 0; i < N; i++) {
        if (A[i] == (i + 1)) {
            count++;
        }
    }

    // Pairs following the first condition
    answer += count * (count - 1) / 2;

    // Calculating number of pairs following
    // the second condition
    for (int i = 0; i < N; i++) {
        if (A[i] > (i + 1) && A[A[i] - 1] == (i + 1)) {
            answer++;
        }
    }

    // Returning answer
    return answer;
}

// Driver Code
int main()
{
    int N = 4;
    int A[] = { 1, 3, 2, 4 };

    // Function call
    int answer = countPairs(N, A);
    cout << answer << endl;
    return 0;
}

Java

/*package whatever //do not write package name here */
import java.io.*;

class GFG {

  // Function to count Number of pairs in
  // array satisfying the given conditions
  static int countPairs(int N, int A[])
  {

    // Variable to store the number of indices such
    // that their value is equal to their position
    int count = 0;

    // Variable to store the total number of pairs
    // following the given condition
    int answer = 0;

    for (int i = 0; i < N; i++) {
      if (A[i] == (i + 1)) {
        count++;
      }
    }

    // Pairs following the first condition
    answer += count * (count - 1) / 2;

    // Calculating number of pairs following
    // the second condition
    for (int i = 0; i < N; i++) {
      if (A[i] > (i + 1) && A[A[i] - 1] == (i + 1)) {
        answer++;
      }
    }

    // Returning answer
    return answer;
  }

  public static void main (String[] args)
  {

    int N = 4;
    int A[] = { 1, 3, 2, 4 };

    // Function call
    int answer = countPairs(N, A);
    System.out.println(answer);
  }
}

// This code is contributed by aadityapburujwale

Python3

# Python3 code for the above approach

# Function to count Number of pairs
# in array satisfying the given conditions
def countpairs(n,a)->int:

    # Variable to store the number of indices
    # such that their value is equal to their position
    count = 0

    # Variable to store the total number
    # of pairs following the given condition
    answer = 0

    for i in range(0,n):
        if(a[i] == i+1):
            count += 1

    # Pairs following the first condition
    answer += ((count)*(count-1))//2

    # Calculating number of pairs following the second condition
    for i in range(0,n):
        if(a[i] > (i+1) and a[a[i]-1] == (i+1)):
            answer += 1

    # Returning answer
    return answer

# Driver Code
if __name__ == '__main__':
    n = 4
    a = [1,3,2,4]

    # Function call
    ans = countpairs(n,a)
    print(ans)

    # This code is contributed by ajaymakvana.

C#

// C# program for above approach
using System;
class GFG
{

  // Function to count Number of pairs in
  // array satisfying the given conditions
  static int countPairs(int N, int[] A)
  {

    // Variable to store the number of indices such
    // that their value is equal to their position
    int count = 0;

    // Variable to store the total number of pairs
    // following the given condition
    int answer = 0;

    for (int i = 0; i < N; i++) {
      if (A[i] == (i + 1)) {
        count++;
      }
    }

    // Pairs following the first condition
    answer += count * (count - 1) / 2;

    // Calculating number of pairs following
    // the second condition
    for (int i = 0; i < N; i++) {
      if (A[i] > (i + 1) && A[A[i] - 1] == (i + 1)) {
        answer++;
      }
    }

    // Returning answer
    return answer;
  }

// Driver Code
public static void Main()
{
    int N = 4;
    int[] A = { 1, 3, 2, 4 };

    // Function call
    int answer = countPairs(N, A);
    Console.Write(answer);
}
}

// This code is contributed by code_hunt.

Javascript

<script>
// Javascript  code for the above approach

// Function to count Number of pairs in
// array satisfying the given conditions
function countPairs( N,A)
{
    // Variable to store the number of indices such
    // that their value is equal to their position
    let count = 0;

    // Variable to store the total number of pairs
    // following the given condition
    let answer = 0;

    for (let i = 0; i < N; i++) {
        if (A[i] == (i + 1)) {
            count++;
        }
    }

    // Pairs following the first condition
    answer += count * (count - 1) / 2;

    // Calculating number of pairs following
    // the second condition
    for ( let i = 0; i < N; i++) {
        if (A[i] > (i + 1) && A[A[i] - 1] == (i + 1)) {
            answer++;
        }
    }

    // Returning answer
    return answer;
}

// Driver Code

    let N = 4;
    let A = [ 1, 3, 2, 4 ];

    // Function call
    let answer = countPairs(N, A);
    document.write(answer);

    // This code is contributed by satwik4409.
    </script>

Output

2

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