# Remove all occurrences of a word from a given string using Z-algorithm

• Difficulty Level : Hard
• Last Updated : 07 Nov, 2022

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”

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 using namespace std; // Function to fill the Z-array for strvoid 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 than 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 strstring 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 Codeint main(){    string str        = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching";    string word = "km";     cout << goodStr(str, word);    return 0;}

## Java

 // Java program for the above approachimport 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 than 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 strdef 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 than 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 Codeif __name__ == "__main__":     st = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching"    word = "km"     print(goodStr(st, word))     # This code is contributed by chitranayal.

## C#

 // C# program for the above approachusing 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 than 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.



## C++

 // C++ program for the above approach#include using namespace std; // Function to fill the Z-array for strvoid 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 than 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 strstring 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 Codeint main(){    string str        = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching";    string word = "km";     cout << goodStr(str, word);    return 0;}

## Java

 // Java program for the above approachimport 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 than 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 strdef 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 than 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 Codeif __name__ == "__main__":     st = "Z-kmalgorithmkmiskmkmkmhelpfulkminkmsearching"    word = "km"     print(goodStr(st, word))     # This code is contributed by chitranayal.

## C#

 // C# program for the above approachusing 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 than 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.

## Javascript



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

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