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Minimize operations to delete all elements of permutation A by removing a subsequence having order as array B

  • Last Updated : 24 Oct, 2021

Given two permutation arrays A[] and B[] of the first N Natural Numbers, the task is to find the minimum number of operations required to remove all array elements A[] such that in each operation remove the subsequence of array elements A[] whose order is the same as in the array B[].

Example:

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Input: A[] = { 4, 2, 1, 3 }, B[] = { 1, 3, 2, 4 }
Output: 3
Explanation:
The given example can be solved by following the given steps:



  1. During the 1st operation, integers at index 2 and 3 in the array A[] can be deleted. Hence, the array A[] = {4, 2}.
  2. During the 2st operation, integer at index 1 in the array A[] can be deleted. Hence, the array A[] = {4}.
  3. During the 3rd operation, integer at index 0 in the array A[] can be deleted. Hence, the array A[] = {}.

The order in which the elements are deleted is {1, 3, 2, 4} which is equal to B. Hence a minimum of 3 operations is required.

Input: A[] = {2, 4, 6, 1, 5, 3}, B[] = {6, 5, 4, 2, 3, 1}
Output: 4

Approach: The given problem can be solved using the steps discussed below:

  1. Create two variables i and j, where i keeps track of the index of the current element of B that is to be deleted next and j keeps track of the current element of A. Initially, both i=0 and j=0.
  2. Traverse the permutation array A[] using j for all values of j in range [0, N-1]. If A[j] = B[i], increment the value of i by 1 and continue traversing the array A[].
  3. After the array A[] has been traversed completely, increment the value of cnt variable which maintains the count of required operations.
  4. Repeat steps 2 and 3 till i<N.
  5. After completing the above steps, the value stored in cnt is the required answer.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the minimum number of
// operations to delete all elements of
// permutation A in order described by B
int minOperations(int A[], int B[], int N)
{
    // Stores the count of operations
    int cnt = 0;
 
    // Stores the index of current integer
    // in B to be deleted
    int i = 0;
 
    // Loop to iterate over all values of B
    while (i < N) {
 
        // Stores the current index in A
        int j = 0;
 
        // Iterate over all values A
        while (j < N && i < N) {
 
            // If current integer of B and A
            // equal, increment the index of
            // the current integer of B
            if (B[i] == A[j]) {
                i++;
            }
            j++;
        }
 
        // As the permutation A has been
        // traversed completelly, increment
        // the count of operations by 1
        cnt++;
    }
 
    // Return Answer
    return cnt;
}
 
// Driver Code
int main()
{
    int A[] = { 2, 4, 6, 1, 5, 3 };
    int B[] = { 6, 5, 4, 2, 3, 1 };
    int N = sizeof(A) / sizeof(A[0]);
 
    cout << minOperations(A, B, N);
 
    return 0;
}

Java




// Java program for the above approach
import java.util.*;
 
class GFG{
 
// Function to find the minimum number of
// operations to delete all elements of
// permutation A in order described by B
static int minOperations(int A[], int B[], int N)
{
   
    // Stores the count of operations
    int cnt = 0;
 
    // Stores the index of current integer
    // in B to be deleted
    int i = 0;
 
    // Loop to iterate over all values of B
    while (i < N) {
 
        // Stores the current index in A
        int j = 0;
 
        // Iterate over all values A
        while (j < N && i < N) {
 
            // If current integer of B and A
            // equal, increment the index of
            // the current integer of B
            if (B[i] == A[j]) {
                i++;
            }
            j++;
        }
 
        // As the permutation A has been
        // traversed completely, increment
        // the count of operations by 1
        cnt++;
    }
 
    // Return Answer
    return cnt;
}
 
// Driver Code
public static void main(String[] args)
{
    int A[] = { 2, 4, 6, 1, 5, 3 };
    int B[] = { 6, 5, 4, 2, 3, 1 };
    int N = A.length;
 
    System.out.print(minOperations(A, B, N));
 
}
}
 
// This code is contributed by 29AjayKumar

Python3




# Python 3 program for the above approach
 
# Function to find the minimum number of
# operations to delete all elements of
# permutation A in order described by B
def minOperations(A, B, N):
   
    # Stores the count of operations
    cnt = 0
 
    # Stores the index of current integer
    # in B to be deleted
    i = 0
 
    # Loop to iterate over all values of B
    while(i < N):
       
        # Stores the current index in A
        j = 0
 
        # Iterate over all values A
        while (j < N and i < N):
 
            # If current integer of B and A
            # equal, increment the index of
            # the current integer of B
            if (B[i] == A[j]):
                i += 1
            j += 1
 
        # As the permutation A has been
        # traversed completely, increment
        # the count of operations by 1
        cnt += 1
 
    # Return Answer
    return cnt
 
# Driver Code
if __name__ == '__main__':
    A = [2, 4, 6, 1, 5, 3]
    B = [6, 5, 4, 2, 3, 1]
    N = len(A)
 
    print(minOperations(A, B, N))
 
    # This code is contributed by SURENDRA_GANGWAR.

C#




// C# program for the above approach
using System;
 
class GFG {
 
    // Function to find the minimum number of
    // operations to delete all elements of
    // permutation A in order described by B
    static int minOperations(int[] A, int[] B, int N)
    {
 
        // Stores the count of operations
        int cnt = 0;
 
        // Stores the index of current integer
        // in B to be deleted
        int i = 0;
 
        // Loop to iterate over all values of B
        while (i < N) {
 
            // Stores the current index in A
            int j = 0;
 
            // Iterate over all values A
            while (j < N && i < N) {
 
                // If current integer of B and A
                // equal, increment the index of
                // the current integer of B
                if (B[i] == A[j]) {
                    i++;
                }
                j++;
            }
 
            // As the permutation A has been
            // traversed completely, increment
            // the count of operations by 1
            cnt++;
        }
 
        // Return Answer
        return cnt;
    }
 
    // Driver Code
    public static void Main(string[] args)
    {
        int[] A = { 2, 4, 6, 1, 5, 3 };
        int[] B = { 6, 5, 4, 2, 3, 1 };
        int N = A.Length;
 
        Console.WriteLine(minOperations(A, B, N));
    }
}
 
// This code is contributed by ukasp.

Javascript




<script>
       // JavaScript Program to implement
       // the above approach
 
 
       // Function to find the minimum number of
       // operations to delete all elements of
       // permutation A in order described by B
       function minOperations(A, B, N) {
           // Stores the count of operations
           let cnt = 0;
 
           // Stores the index of current integer
           // in B to be deleted
           let i = 0;
 
           // Loop to iterate over all values of B
           while (i < N) {
 
               // Stores the current index in A
               let j = 0;
 
               // Iterate over all values A
               while (j < N && i < N) {
 
                   // If current integer of B and A
                   // equal, increment the index of
                   // the current integer of B
                   if (B[i] == A[j]) {
                       i++;
                   }
                   j++;
               }
 
               // As the permutation A has been
               // traversed completely, increment
               // the count of operations by 1
               cnt++;
           }
 
           // Return Answer
           return cnt;
       }
 
       // Driver Code
 
       let A = [2, 4, 6, 1, 5, 3];
       let B = [6, 5, 4, 2, 3, 1];
       let N = A.length;
 
       document.write(minOperations(A, B, N));
 
    // This code is contributed by Potta Lokesh
 
   </script>
Output: 
4

 

Time Complexity: O(N2)
Auxiliary Space: O(1)




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