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Minimize removals to remove another string as a subsequence of a given string
  • Difficulty Level : Expert
  • Last Updated : 08 Mar, 2021
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Given two strings str and X of length N and M respectively, the task is to find the minimum characters required to be removed from the string str such that string str doesn’t contain the string X as a subsequence.

Examples:

Input: str = “btagd”, X = “bad”
Output: 1
Explanation:
String “btag” has does not contain “bad” as a subsequence. Therefore, only one removal is required.

Input: str = “bbaaddd”, X = “bad”
Output: 2

 

Approach: This problem can be solved by Dynamic Programming. Follow the steps below to solve the problem:

  • Traverse the string.
  • Initialize a 2D array dp[N][M], where N is the length of string str and M is the length of string X.
  • dp[i][j] represents the minimum number of characters removal required in the substring str[0, i] such that there is no occurrence of substring X[0, j] as a subsequence.
  • The two transition states are :
    • If str[i] is equal to X[j],
      • Case 1 : Remove the character str[i]
      • Case 2 : Maintain the character str[i], by ensuring that there is no occurrence of X[0, j-1] as a subsequence in str[0, i]
      • Update dp[i][j] = min(dp[i − 1][j − 1], dp[i − 1][j] + 1)
    • If str[i] is not equal to X[j], str[i] can be kept as it is.
      •  Update dp[i][j] = dp[i – 1][j]
  • Print the minimum removals i.e, dp[N – 1][M – 1]

Below is the implementation of the above approach:



C++




// C++ implementation of
// the above approach
#include <bits/stdc++.h>
 
using namespace std;
 
// Function to print the minimum number of
// character removals required to remove X
// as a subsequence from the string str
void printMinimumRemovals(string str, string X)
{
 
    // Length of the string str
    int N = str.size();
 
    // Length of the string X
    int M = X.size();
 
    // Stores the dp states
    int dp[N][M] = {};
 
    // Fill first row of dp[][]
    for (int j = 0; j < M; j++) {
 
        // If X[j] matches with str[0]
        if (str[0] == X[j]) {
 
            dp[0][j] = 1;
        }
    }
 
    for (int i = 1; i < N; i++) {
 
        for (int j = 0; j < M; j++) {
 
            // If str[i] is equal to X[j]
            if (str[i] == X[j]) {
 
                // Update state after removing str[i[
                dp[i][j] = dp[i - 1][j] + 1;
 
                // Update state after keeping str[i]
                if (j != 0)
                    dp[i][j]
                        = min(dp[i][j], dp[i - 1][j - 1]);
            }
 
            // If str[i] is not equal to X[j]
            else {
 
                dp[i][j] = dp[i - 1][j];
            }
        }
    }
 
    // Print the minimum number
    // of characters removals
    cout << dp[N - 1][M - 1];
}
 
// Driver Code
int main()
{
    // Input
    string str = "btagd";
    string X = "bad";
 
    // Function call to get minimum
    // number of character removals
    printMinimumRemovals(str, X);
 
    return 0;
}

Java




// Java program for the above approach
import java.io.*;
import java.lang.*;
import java.util.*;
class GFG {
 
  // Function to print the minimum number of
  // character removals required to remove X
  // as a subsequence from the string str
  static void printMinimumRemovals(String str, String X)
  {
 
    // Length of the string str
    int N = str.length();
 
    // Length of the string X
    int M = X.length();
 
    // Stores the dp states
    int dp[][] = new int[N][M];
 
    // Fill first row of dp[][]
    for (int j = 0; j < M; j++) {
 
      // If X[j] matches with str[0]
      if (str.charAt(0) == X.charAt(j)) {
 
        dp[0][j] = 1;
      }
    }
 
    for (int i = 1; i < N; i++) {
 
      for (int j = 0; j < M; j++) {
 
        // If str[i] is equal to X[j]
        if (str.charAt(i) == X.charAt(j)) {
 
          // Update state after removing str[i[
          dp[i][j] = dp[i - 1][j] + 1;
 
          // Update state after keeping str[i]
          if (j != 0)
            dp[i][j] = Math.min(
            dp[i][j], dp[i - 1][j - 1]);
        }
 
        // If str[i] is not equal to X[j]
        else {
 
          dp[i][j] = dp[i - 1][j];
        }
      }
    }
 
    // Print the minimum number
    // of characters removals
    System.out.println(dp[N - 1][M - 1]);
  }
 
  // Driver code
  public static void main(String[] args)
  {
    // Input
    String str = "btagd";
    String X = "bad";
 
    // Function call to get minimum
    // number of character removals
    printMinimumRemovals(str, X);
  }
}
 
// This code is contributed by Kingash.

Python3




# Python 3 implementation of
# the above approach
 
# Function to print the minimum number of
# character removals required to remove X
# as a subsequence from the string str
def printMinimumRemovals(s, X):
 
    # Length of the string str
    N = len(s)
 
    # Length of the string X
    M = len(X)
 
    # Stores the dp states
    dp = [[0 for x in range(M)] for y in range(N)]
 
    # Fill first row of dp[][]
    for j in range(M):
       
        # If X[j] matches with str[0]
        if (s[0] == X[j]):
            dp[0][j] = 1
 
    for i in range(1, N):
        for j in range(M):
           
            # If s[i] is equal to X[j]
            if (s[i] == X[j]):
 
                # Update state after removing str[i[
                dp[i][j] = dp[i - 1][j] + 1
 
                # Update state after keeping str[i]
                if (j != 0):
                    dp[i][j] = min(dp[i][j], dp[i - 1][j - 1])
 
            # If str[i] is not equal to X[j]
            else:
                dp[i][j] = dp[i - 1][j]
 
    # Print the minimum number
    # of characters removals
    print(dp[N - 1][M - 1])
 
# Driver Code
if __name__ == "__main__":
 
    # Input
    s = "btagd"
    X = "bad"
 
    # Function call to get minimum
    # number of character removals
    printMinimumRemovals(s, X)
 
    # This code is contributed by ukasp.

C#




// C# program for above approach
using System;
public class GFG
{
 
  // Function to print the minimum number of
  // character removals required to remove X
  // as a subsequence from the string str
  static void printMinimumRemovals(string str, string X)
  {
 
    // Length of the string str
    int N = str.Length;
 
    // Length of the string X
    int M = X.Length;
 
    // Stores the dp states
    int[,] dp = new int[N, M];
 
    // Fill first row of dp[][]
    for (int j = 0; j < M; j++) {
 
      // If X[j] matches with str[0]
      if (str[0] == X[j]) {
 
        dp[0, j] = 1;
      }
    }
 
    for (int i = 1; i < N; i++) {
 
      for (int j = 0; j < M; j++) {
 
        // If str[i] is equal to X[j]
        if (str[i] == X[j]) {
 
          // Update state after removing str[i[
          dp[i, j] = dp[i - 1, j] + 1;
 
          // Update state after keeping str[i]
          if (j != 0)
            dp[i, j] = Math.Min(
            dp[i, j], dp[i - 1, j - 1]);
        }
 
        // If str[i] is not equal to X[j]
        else {
 
          dp[i, j] = dp[i - 1, j];
        }
      }
    }
 
    // Print the minimum number
    // of characters removals
    Console.WriteLine(dp[N - 1, M - 1]);
  }
 
// Driver code
public static void Main(String[] args)
{
   
    // Input
    string str = "btagd";
    string X = "bad";
 
    // Function call to get minimum
    // number of character removals
    printMinimumRemovals(str, X);
}
}
 
// This code is contributed by sanjoy_62.

 
 

Output: 
1

 

 

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

 

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