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Remove all occurrences of a word from a given string using Z-algorithm
  • Difficulty Level : Hard
  • Last Updated : 05 Feb, 2021
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Given two strings str of length N and word of length M, the task is to remove all the occurrences of the string word from the string str.

Examples: 
 

Input: str = “asmGeeksasmasmForasmGeeks”, word = “asm” 
Output: GeeksForGeeks 
Explanation: 
Removing “asm” from the string, str modifies str to GeeksForGeeks

Input: str = “Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching”, word = “km” 
Output: Z-algorithmishelpfulinsearching

Explanation: 
Removing “km” from the string, str modifies str to “Z-algorithmishelpfulinsearching”.



 
 

 

 

 

Naive Approach: The simplest approach to solve this problem is to iterate over the characters of the string str. For every index, check if a substring can be found whose starting index is equal to the current index and the substring is equal to the string, word. If found to be true, then remove the substring. Finally, print the string. 
Time Complexity: O(N2) 
Auxiliary Space: O(1)

STL-based Approach: Remove all the occurrence of the string word from the string str using replace() method. 
Time Complexity: O(N2) 
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized using Z-algorithm. Follow the steps below to solve the problem:

 

  • Initialize a string, say res, to store the string by removing the words from the given string str.
  • Initialize an array, say Z[], to store the Z-value of the string.
  • Find all occurrences of the string word in the given string str using Z-algorithm.
  • Finally, traverse the array Z[] and check if z[i + length(word) + 1] is equal to length(word) or not. If found to be true, then update i += length(word) – 1.
  • Otherwise, append current character into the string res.

 

Below is the implementation of the above approach:

 

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to fill the Z-array for str
void getZarr(string str, int Z[])
{
    int n = str.length();
    int k;
 
    // L Stores start index of window
    // which matches with prefix of str
    int L = 0;
 
    // R Stores end index of window
    // which matches with prefix of str
    int R = 0;
 
    // Iterate over the characters of str
    for (int i = 1; i < n; ++i) {
 
        // If i is greater thn R
        if (i > R) {
 
            // Update L and R
            L = R = i;
 
            // If substring match with prefix
            while (R < n && str[R - L] == str[R]) {
 
                // Update R
                R++;
            }
 
            // Update Z[i]
            Z[i] = R - L;
 
            // Update R
            R--;
        }
        else {
 
            // Update k
            k = i - L;
 
            // if Z[k] is less than
            // remaining interval
            if (Z[k] < R - i + 1) {
 
                // Update Z[i]
                Z[i] = Z[k];
            }
 
            else {
 
                // Start from R and check manually
                L = i;
                while (R < n && str[R - L] == str[R]) {
 
                    // Update R
                    R++;
                }
 
                // Update Z[i]
                Z[i] = R - L;
 
                // Update R
                R--;
            }
        }
    }
}
 
// Function to remove all the occurrences
// of word from str
string goodStr(string str, string word)
{
    // Create concatenated string "P$T"
    string concat = word + "$" + str;
    int l = concat.length();
 
    // Store Z array of concat
    int Z[l];
 
    getZarr(concat, Z);
 
    // Stores string, str by removing all
    // the occurrences of word from str
    string res;
 
    // Stores length of word
    int pSize = word.size();
 
    // Traverse the array, Z[]
    for (int i = 0; i < l; ++i) {
 
        // if Z[i + pSize + 1] equal to
        // length of word
        if (i + pSize < l - 1 && Z[i + pSize + 1] == pSize) {
 
            // Update i
            i += pSize - 1;
        }
        else if (i < str.length()) {
            res += str[i];
        }
    }
    return res;
}
 
// Driver Code
int main()
{
    string str
        = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching";
    string word = "km";
 
    cout << goodStr(str, word);
    return 0;
}

Java




// Java program for the above approach
import java.util.*;
class GFG
{
 
  // Function to fill the Z-array for str
  static void getZarr(String str, int Z[])
  {
    int n = str.length();
    int k;
 
    // L Stores start index of window
    // which matches with prefix of str
    int L = 0;
 
    // R Stores end index of window
    // which matches with prefix of str
    int R = 0;
 
    // Iterate over the characters of str
    for (int i = 1; i < n; ++i)
    {
 
      // If i is greater thn R
      if (i > R)
      {
 
        // Update L and R
        L = R = i;
 
        // If subString match with prefix
        while (R < n && str.charAt(R - L) ==
               str.charAt(R))
        {
 
          // Update R
          R++;
        }
 
        // Update Z[i]
        Z[i] = R - L;
 
        // Update R
        R--;
      }
      else
      {
 
        // Update k
        k = i - L;
 
        // if Z[k] is less than
        // remaining interval
        if (Z[k] < R - i + 1)
        {
 
          // Update Z[i]
          Z[i] = Z[k];
        }
 
        else
        {
 
          // Start from R and check manually
          L = i;
          while (R < n && str.charAt(R - L) ==
                 str.charAt(R))
          {
 
            // Update R
            R++;
          }
 
          // Update Z[i]
          Z[i] = R - L;
 
          // Update R
          R--;
        }
      }
    }
  }
 
  // Function to remove all the occurrences
  // of word from str
  static String goodStr(String str, String word)
  {
    // Create concatenated String "P$T"
    String concat = word + "$" + str;
    int l = concat.length();
 
    // Store Z array of concat
    int []Z = new int[l];
 
    getZarr(concat, Z);
 
    // Stores String, str by removing all
    // the occurrences of word from str
    String res="";
 
    // Stores length of word
    int pSize = word.length();
 
    // Traverse the array, Z[]
    for (int i = 0; i < l; ++i) {
 
      // if Z[i + pSize + 1] equal to
      // length of word
      if (i + pSize < l - 1 && Z[i + pSize + 1] == pSize) {
 
        // Update i
        i += pSize - 1;
      }
      else if (i < str.length()) {
        res += str.charAt(i);
      }
    }
    return res;
  }
 
  // Driver Code
  public static void main(String[] args)
  {
    String str
      = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching";
    String word = "km";
 
    System.out.print(goodStr(str, word));
  }
}
 
// This code is contributed by 29AjayKumar

Python3




# Python 3 program for the above approach
 
# Function to fill the Z-array for str
def getZarr(st, Z):
    n = len(st)
    k = 0
 
    # L Stores start index of window
    # which matches with prefix of str
    L = 0
 
    # R Stores end index of window
    # which matches with prefix of str
    R = 0
 
    # Iterate over the characters of str
    for i in range(1, n):
 
        # If i is greater thn R
        if (i > R):
 
            # Update L and R
            L = R = i
 
            # If substring match with prefix
            while (R < n and st[R - L] == st[R]):
 
                # Update R
                R += 1
 
            # Update Z[i]
            Z[i] = R - L
 
            # Update R
            R -= 1
 
        else:
 
            # Update k
            k = i - L
 
            # if Z[k] is less than
            # remaining interval
            if (Z[k] < R - i + 1):
 
                # Update Z[i]
                Z[i] = Z[k]
 
            else:
 
                # Start from R and check manually
                L = i
                while (R < n and st[R - L] == st[R]):
 
                    # Update R
                    R += 1
 
                # Update Z[i]
                Z[i] = R - L
 
                # Update R
                R -= 1
 
# Function to remove all the occurrences
# of word from str
 
 
def goodStr(st, word):
 
    # Create concatenated string "P$T"
    concat = word + "$" + st
    l = len(concat)
 
    # Store Z array of concat
    Z = [0]*l
 
    getZarr(concat, Z)
 
    # Stores string, str by removing all
    # the occurrences of word from str
    res = ""
 
    # Stores length of word
    pSize = len(word)
 
    # Traverse the array, Z[]
    for i in range(l):
 
        # if Z[i + pSize + 1] equal to
        # length of word
        if (i + pSize < l - 1 and Z[i + pSize + 1] == pSize):
 
            # Update i
            i += pSize - 1
 
        elif (i < len(st)):
            res += st[i]
 
    return res
 
# Driver Code
if __name__ == "__main__":
 
    st = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching"
    word = "km"
 
    print(goodStr(st, word))
 
    # This code is contributed by chitranayal.

C#




// C# program for the above approach
using System;
using System.Collections.Generic;
 
public class GFG
{
 
  // Function to fill the Z-array for str
  static void getZarr(string str, int[] Z)
  {
    int n = str.Length;
    int k;
 
    // L Stores start index of window
    // which matches with prefix of str
    int L = 0;
 
    // R Stores end index of window
    // which matches with prefix of str
    int R = 0;
 
    // Iterate over the characters of str
    for (int i = 1; i < n; ++i)
    {
 
      // If i is greater thn R
      if (i > R)
      {
 
        // Update L and R
        L = R = i;
 
        // If subString match with prefix
        while (R < n && str[R - L] ==
               str[R])
        {
 
          // Update R
          R++;
        }
 
        // Update Z[i]
        Z[i] = R - L;
 
        // Update R
        R--;
      }
      else
      {
 
        // Update k
        k = i - L;
 
        // if Z[k] is less than
        // remaining interval
        if (Z[k] < R - i + 1)
        {
 
          // Update Z[i]
          Z[i] = Z[k];
        }
 
        else
        {
 
          // Start from R and check manually
          L = i;
          while (R < n && str[R - L] ==
                 str[R])
          {
 
            // Update R
            R++;
          }
 
          // Update Z[i]
          Z[i] = R - L;
 
          // Update R
          R--;
        }
      }
    }
  }
 
  // Function to remove all the occurrences
  // of word from str
  static string goodStr(string str, string word)
  {
     
    // Create concatenated String "P$T"
    string concat = word + "$" + str;
    int l = concat.Length;
 
    // Store Z array of concat
    int []Z = new int[l];
    getZarr(concat, Z);
 
    // Stores String, str by removing all
    // the occurrences of word from str
    string res="";
 
    // Stores length of word
    int pSize = word.Length;
 
    // Traverse the array, Z[]
    for (int i = 0; i < l; ++i) {
 
      // if Z[i + pSize + 1] equal to
      // length of word
      if (i + pSize < l - 1 && Z[i + pSize + 1] == pSize) {
 
        // Update i
        i += pSize - 1;
      }
      else if (i < str.Length) {
        res += str[i];
      }
    }
    return res;
  }
 
  // Driver Code
  static public void Main()
  {
    string str
      = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching";
    string word = "km";
 
    Console.WriteLine(goodStr(str, word));
  }
}
 
// This code is contributed by sanjoy_62.
Output: 
Z-algorithmishelpfulinsearching

 

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

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