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Minimize elements to be added to a given array such that it contains another given array as its subsequence | Set 2
  • Difficulty Level : Expert
  • Last Updated : 19 Oct, 2020

Given an array A[] consisting of N distinct integers and another array B[] consisting of M integers, the task is to find the minimum number of elements to be added to the array B[] such that the array A[] becomes the subsequence of the array B[].

Examples:

Input: N = 5, M = 6, A[] = {1, 2, 3, 4, 5}, B[] = {2, 5, 6, 4, 9, 12}
Output: 3
Explanation:
Below are the element that are needed to be added:
1) Add 1 before element 2 of B[]
2) Add 3 after element 6 of B[]
3) Add 5 in the last position of B[].
Therefore, the resulting array B[] is {1, 2, 5, 6, 3, 4, 9, 12, 5}.
Hence, A[] is the subsequence of B[] after adding 3 elements.

Input: N = 5, M = 5, A[] = {3, 4, 5, 2, 7}, B[] = {3, 4, 7, 9, 2}
Output:
Explanation: 
Below are the elements that are needed to be added: 
1) Add 5 after element 4. 
2) Add 2 after element 5. 
Therefore, the resulting array B[] is {3, 4, 5, 2, 7, 9, 2}. 
Hence 2 elements are required to be added.

Naive Approach: Refer to the previous post of this article for the simplest approach to solve the problem. 

Time Complexity: O(N * 2M)
Auxiliary Space: O(M + N)

Dynamic Programming Approach: Refer to the previous post of this article for the Longest Common Subsequence based approach. 

Time Complexity: O(N * M)
Auxiliary Space: O(N * M)

Efficient Approach: The idea is similar to finding the Longest Increasing Subsequence(LIS) from the array B[]. Follow the steps below to solve the problem:



  • Consider elements of array B[] which are present in the array A[], and store the indices of each element of the array A[] in a Map
  • Then, find the LIS array subseq[] using Binary Search which consists of the indices in increasing order.
  • Finally, the minimum number of elements to be inserted into array B[] is equal to N – len(LIS), where len(LIS) is calculated using Binary Search in the above steps.

Below is the implementation of the above approach:

C++

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// C++ program for the
// above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to return minimum
// element to be added in array
// B so that array A become
// subsequence of array B
int minElements(int A[], int B[],
                int N, int M)
{
  // Stores indices of the
  // array elements
  map<int, int> map;
 
  // Iterate over the array
  for (int i = 0; i < N; i++)
  {
    // Store the indices of
    // the array elements
    map[A[i]] = i;
  }
 
  // Stores the LIS
  vector<int> subseq;
 
  int l = 0, r = -1;
 
  for (int i = 0; i < M; i++)
  {
    // Check if element B[i]
    // is in array A[]
    if (map.find(B[i]) !=
        map.end())
    {
      int e = map[B[i]];
 
      // Perform Binary Search
      while (l <= r)
      {
        // Find the value of
        // mid m
        int m = l + (r - l) / 2;
 
        // Update l and r
        if (subseq[m] < e)
          l = m + 1;
        else
          r = m - 1;
      }
 
      // If found better element
      // 'e' for pos r + 1
      if (r + 1 < subseq.size())
      {
        subseq[r + 1] = e;
      }
 
      // Otherwise, extend the
      // current subsequence
      else
      {
        subseq.push_back(e);
      }
 
      l = 0;
      r = subseq.size() - 1;
    }
  }
 
  // Return the answer
  return N - subseq.size();
}
 
// Driver code
int main()
{
  // Given arrays
  int A[] = {1, 2, 3, 4, 5};
  int B[] = {2, 5, 6, 4, 9, 12};
 
  int M = sizeof(A) /
          sizeof(A[0]);
  int N = sizeof(B) /
          sizeof(B[0]);
 
  // Function Call
  cout << minElements(A, B,
                      M, N);
 
  return 0;
}
 
// This code is contributed by divyeshrabadiya07

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Java

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// Java program for the above approach
 
import java.util.*;
import java.lang.*;
 
class GFG {
 
    // Function to return minimum element
    // to be added in array B so that array
    // A become subsequence of array B
    static int minElements(
        int[] A, int[] B, int N, int M)
    {
 
        // Stores indices of the
        // array elements
        Map<Integer, Integer> map
            = new HashMap<>();
 
        // Iterate over the array
        for (int i = 0;
             i < A.length; i++) {
 
            // Store the indices of
            // the array elements
            map.put(A[i], i);
        }
 
        // Stores the LIS
        ArrayList<Integer> subseq
            = new ArrayList<>();
 
        int l = 0, r = -1;
 
        for (int i = 0; i < M; i++) {
 
            // Check if element B[i]
            // is in array A[]
            if (map.containsKey(B[i])) {
 
                int e = map.get(B[i]);
 
                // Perform Binary Search
                while (l <= r) {
 
                    // Find the value of
                    // mid m
                    int m = l + (r - l) / 2;
 
                    // Update l and r
                    if (subseq.get(m) < e)
                        l = m + 1;
                    else
                        r = m - 1;
                }
 
                // If found better element
                // 'e' for pos r + 1
                if (r + 1 < subseq.size()) {
                    subseq.set(r + 1, e);
                }
 
                // Otherwise, extend the
                // current subsequence
                else {
                    subseq.add(e);
                }
 
                l = 0;
                r = subseq.size() - 1;
            }
        }
 
        // Return the answer
        return N - subseq.size();
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Given arrays
        int[] A = { 1, 2, 3, 4, 5 };
        int[] B = { 2, 5, 6, 4, 9, 12 };
 
        int M = A.length;
        int N = B.length;
 
        // Function Call
        System.out.println(
            minElements(A, B, M, N));
    }
}

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Python3

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# Python3 program for the above approach
 
# Function to return minimum element
# to be added in array B so that array
# A become subsequence of array B
def minElements(A, B, N, M):
     
    # Stores indices of the
    # array elements
    map = {}
 
    # Iterate over the array
    for i in range(len(A)):
 
        # Store the indices of
        # the array elements
        map[A[i]] = i
  
    # Stores the LIS
    subseq = []
 
    l = 0
    r = -1
 
    for i in range(M):
 
        # Check if element B[i]
        # is in array A[]
        if B[i] in map:
            e = map[B[i]]
 
            # Perform Binary Search
            while (l <= r):
 
                # Find the value of
                # mid m
                m = l + (r - l) // 2
 
                # Update l and r
                if (subseq[m] < e):
                    l = m + 1
                else:
                    r = m - 1
 
            # If found better element
            # 'e' for pos r + 1
            if (r + 1 < len(subseq)):
                subseq[r + 1]= e
 
            # Otherwise, extend the
            # current subsequence
            else:
                subseq.append(e)
 
            l = 0
            r = len(subseq) - 1
 
    # Return the answer
    return N - len(subseq)
 
# Driver Code
if __name__ == '__main__':
     
    # Given arrays
    A = [ 1, 2, 3, 4, 5 ]
    B = [ 2, 5, 6, 4, 9, 12 ]
 
    M = len(A)
    N = len(B)
 
    # Function call
    print(minElements(A, B, M, N))
 
# This code is contributed by mohit kumar 29

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C#

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// C# program for
// the above approach
using System;
using System.Collections.Generic;
class GFG{
 
// Function to return minimum element
// to be added in array B so that array
// A become subsequence of array B
static int minElements(int[] A, int[] B,
                       int N, int M)
{
  // Stores indices of the
  // array elements
  Dictionary<int,
             int> map = new Dictionary<int,
                                       int>();
   
  // Iterate over the array
  for (int i = 0;
           i < A.Length; i++)
  {
    // Store the indices of
    // the array elements
    map.Add(A[i], i);
  }
 
  // Stores the LIS
  List<int> subseq = new List<int>();
 
  int l = 0, r = -1;
 
  for (int i = 0; i < M; i++)
  {
    // Check if element B[i]
    // is in array []A
    if (map.ContainsKey(B[i]))
    {
      int e = map[B[i]];
 
      // Perform Binary Search
      while (l <= r)
      {
        // Find the value of
        // mid m
        int m = l + (r - l) / 2;
 
        // Update l and r
        if (subseq[m] < e)
          l = m + 1;
        else
          r = m - 1;
      }
 
      // If found better element
      // 'e' for pos r + 1
      if (r + 1 < subseq.Count)
      {
        subseq[r + 1] = e;
      }
 
      // Otherwise, extend the
      // current subsequence
      else
      {
        subseq.Add(e);
      }
 
      l = 0;
      r = subseq.Count - 1;
    }
  }
 
  // Return the answer
  return N - subseq.Count;
}
 
// Driver Code
public static void Main(String[] args)
{
  // Given arrays
  int[] A = {1, 2, 3, 4, 5};
  int[] B = {2, 5, 6, 4, 9, 12};
 
  int M = A.Length;
  int N = B.Length;
 
  // Function Call
  Console.WriteLine(minElements(A, B,
                                M, N));
}
}
 
// This code is contributed by Princi Singh

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Output: 

3






 

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

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