Minimum characters to be replaced to remove the given substring

Last Updated : 28 Mar, 2023

Given two strings str1 and str2. The task is to find the minimum number of characters to be replaced by $ in string str1 such that str1 does not contain string str2 as any substring.

Examples:

Input: str1 = "intellect", str2 = "tell"
Output: 1
4th character of string "str1" can be replaced by $
such that "int$llect" it does not contain "tell" 
as a substring.

Input: str1 = "google", str2 = "apple"
Output: 0

Approach is similar to Searching for Patterns | Set 1 (Naive Pattern Searching).

The idea is to find the leftmost occurrence of the string ‘str2’ in the string ‘str1’. If all the characters of ‘str1’ match with ‘str2’, we will replace (or increment our answer with one) the last symbol of occurrence and increment the index of string ‘str1’, such that it checks again for the substring after the replaced character(that is index i will be equal to i+length(b)-1).

Below is the implementation of the above approach:

C++

// C++ implementation of above approach
#include <bits/stdc++.h>
using namespace std;
 
// function to calculate minimum
// characters to replace
int replace(string A, string B)
{
 
    int n = A.length(), m = B.length();
    int count = 0, i, j;
 
    for (i = 0; i < n; i++) {
        for (j = 0; j < m; j++) {
 
            // mismatch occurs
            if (A[i + j] != B[j]) 
                break;
        }
 
        // if all characters matched, i.e,
        // there is a substring of 'a' which
        // is same as string 'b'
        if (j == m) {
            count++;
 
             // increment i to index m-1 such that
            // minimum characters are replaced
            // in 'a'
            i += m - 1;
            
        }
    }
 
    return count;
}
 
// Driver Code
int main()
{
    string str1 = "aaaaaaaa";
    string str2 = "aaa";
 
    cout << replace(str1 , str2);
 
  return 0;
}

Java

// Java implementation of
// above approach
import java.io.*;
 
// function to calculate minimum
// characters to replace
class GFG
{
static int replace(String A, String B)
{
 
    int n = A.length(), m = B.length();
    int count = 0, i, j;
 
    for (i = 0; i < n; i++)
    {
        for (j = 0; j < m; j++) 
        {
 
            // mismatch occurs
            if(i + j >= n)
            break;
            else if (A.charAt(i + j) != B.charAt(j)) 
                break;
        }
 
        // if all characters matched, i.e,
        // there is a substring of 'a' which
        // is same as string 'b'
        if (j == m) 
        {
            count++;
 
            // increment i to index m-1 such that
            // minimum characters are replaced
            // in 'a'
            i += m - 1;
             
        }
    }
 
    return count;
}
 
// Driver Code
public static void main(String args[])
{
    String str1 = "aaaaaaaa";
    String str2 = "aaa";
 
    System.out.println(replace(str1 , str2));
}
}
 
// This code is contributed by Subhadeep

Python3

# Python3 implementation of the 
# above approach 
 
# Function to calculate minimum 
# characters to replace 
def replace(A, B): 
 
    n, m = len(A), len(B) 
    count, i = 0, 0
    while i < n: 
         
        j = 0
        while j < m: 
             
            # mismatch occurs 
            if i + j >= n or A[i + j] != B[j]:
                break
             
            j += 1
             
        # If all characters matched, i.e, 
        # there is a substring of 'a' which 
        # is same as string 'b' 
        if j == m:
            count += 1
                 
            # increment i to index m-1 such that 
            # minimum characters are replaced 
            # in 'a' 
            i += m - 1
             
        i += 1
             
    return count 
 
# Driver Code 
if __name__ == "__main__": 
 
    str1 = "aaaaaaaa"
    str2 = "aaa"
 
    print(replace(str1 , str2)) 
 
# This code is contributed by Rituraj Jain

C#

// C# implementation of above approach 
using System;
 
// function to calculate minimum 
// characters to replace 
class GFG
{
public static int replace(string A, 
                          string B)
{
 
    int n = A.Length, m = B.Length;
    int count = 0, i, j;
 
    for (i = 0; i < n; i++)
    {
        for (j = 0; j < m; j++)
        {
 
            // mismatch occurs 
            if (i + j >= n)
            {
            break;
            }
            else if (A[i + j] != B[j])
            {
                break;
            }
        }
 
        // if all characters matched, i.e, 
        // there is a substring of 'a' 
        // which is same as string 'b' 
        if (j == m)
        {
            count++;
 
            // increment i to index m-1 
            // such that minimum characters 
            // are replaced in 'a' 
            i += m - 1;
 
        }
    }
 
    return count;
}
 
// Driver Code 
public static void Main(string[] args)
{
    string str1 = "aaaaaaaa";
    string str2 = "aaa";
 
    Console.WriteLine(replace(str1, str2));
}
}
 
// This code is contributed 
// by Shrikant13

PHP

<?php
// PHP implementation of above approach 
 
// function to calculate minimum 
// characters to replace 
function replace($A, $B)
{
    $n = strlen($A);
    $m = strlen($B);
    $count = 0;
 
    for ($i = 0; $i < $n; $i++)
    {
        for ($j = 0; $j < $m; $j++)
        {
 
            // mismatch occurs 
            if ($i + $j >= $n)
            {
                break;
            }
            else if ($A[$i + $j] != $B[$j])
            {
                break;
            }
        }
 
        // if all characters matched, i.e, 
        // there is a substring of 'a' 
        // which is same as string 'b' 
        if ($j == $m)
        {
            $count++;
 
            // increment i to index m-1 
            // such that minimum characters 
            // are replaced in 'a' 
            $i = $i + $m - 1;
 
        }
    }
 
    return $count;
}
 
// Driver Code 
$str1 = "aaaaaaaa";
$str2 = "aaa";
 
echo (replace($str1, $str2));
 
// This code is contributed
// by Kirti_Mangal
?>

Javascript

<script>
// JavaScript implementation of above approach
 
// function to calculate minimum
// characters to replace
function replace(A,B)
{
 
    let n = A.length, m = B.length;
    let count = 0, i, j;
 
    for (i = 0; i < n; i++) {
        for (j = 0; j < m; j++) {
 
            // mismatch occurs
            if (A[i + j] != B[j]) 
                break;
        }
 
        // if all characters matched, i.e,
        // there is a substring of 'a' which
        // is same as string 'b'
        if (j == m) {
            count++;
 
             // increment i to index m-1 such that
            // minimum characters are replaced
            // in 'a'
            i += m - 1;
            
        }
    }
 
    return count;
}
 
// Driver Code
 
const str1 = "aaaaaaaa";
const str2 = "aaa";
 
document.write(replace(str1 , str2));
 
// This code is contributed by shinjanpatra.
</script>

Output

Time Complexity: O(len1 * len2), where len1 is the length of first string and len2 is the length of second string.
Auxiliary Space: O(1) because it is using constant space for variable

Also, this problem can be solved directly by using Python’s in-built function-string1.count(string2)

Implementation:

C++

#include <iostream>
#include <string>
using namespace std;
 
int main() {
    string str1 = "aaaaaaaa";
    string str2 = "aaa";
 
    // Using the find function to count the number
  // of non-overlapping occurrences of str2 in str1
    int answer = 0;
    size_t pos = str1.find(str2);
    while (pos != string::npos) {
        answer++;
        pos = str1.find(str2, pos + str2.size());
    }
 
    cout << answer << endl;
}

Java

// Java program to find minimum numbers
// of characters to be replaced to
// remove the given substring
public class Main {
    public static void main(String[] args) {
        String str1 = "aaaaaaaa";
        String str2 = "aaa";
 
        // inbuilt function
        int answer = str1.split(str2, -1).length - 1;
        System.out.println(answer);
    }
}

Python3

#Python program to find minimum numbers
# of characters to be replaced to
#remove the given substring
str1 = "aaaaaaaa"
str2 = "aaa"
 
# inbuilt function
answer = str1.count(str2)
print(answer)

C#

// C# program to find minimum numbers
// of characters to be replaced to
// remove the given substring
using System;
 
class Program {
    static void Main(string[] args)
    {
        string str1 = "aaaaaaaa";
        string str2 = "aaa"; // inbuilt function
        int answer = str1.Split(new[] { str2 },
                                StringSplitOptions.None)
                         .Length
                     - 1;
        Console.WriteLine(answer);
    }
}

Javascript

// JavaScript program to find minimum numbers
// of characters to be replaced to
//remove the given substring
let str1 = "aaaaaaaa";
let str2 = "aaa";
 
// inbuilt function
let answer = str1.split(str2).length - 1;
console.log(answer);
 
// This code is contributed by codebraxnzt

Output

Time Complexity: O(len1 * len2), where len1 is the length of first string and len2 is the length of second string.
Auxiliary Space: O(1) because it is using constant space for variable