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Minimum changes required to make first string substring of second string
• Difficulty Level : Medium
• Last Updated : 06 May, 2021

Given two strings S1 and S2 ( size of S1 <= Size of S2 ). The task is to find the minimum number of characters to be replaced in the string S2, such that the string S1 is a substring of S2.
Examples

```Input : S1 = cdef, S2 = abbdef
Output : 1

Input : S1 = gfg, S2 = fgg
Output : 2 ```

Approach:

1. Traverse the string S2
• From every index in S2, check the number of mis-matching characters in substring of length of S1
• Store and update the minimum of prev and current mis-match in ans
2. Return ans.

Below is the implementation of the above approach:

## C++

 `// CPP program to find the minimum number of``// characters to be replaced in string S2, such``// that S1 is a substring of S2` `#include ``using` `namespace` `std;` `// Function to find the minimum number of``// characters to be replaced in string S2, such``// that S1 is a substring of S2``int` `minimumChar(string S1, string S2)``{``    ``// Get the sizes of both strings``    ``int` `n = S1.size(), m = S2.size();` `    ``int` `ans = INT_MAX;` `    ``// Traverse the string S2``    ``for` `(``int` `i = 0; i < m - n + 1; i++) {``        ``int` `minRemovedChar = 0;` `        ``// From every index in S2, check the number of``        ``// mis-matching characters in substring of``        ``// length of S1``        ``for` `(``int` `j = 0; j < n; j++) {``            ``if` `(S1[j] != S2[i + j]) {``                ``minRemovedChar++;``            ``}``        ``}` `        ``// Take minimum of prev and current mis-match``        ``ans = min(minRemovedChar, ans);``    ``}` `    ``// return answer``    ``return` `ans;``}` `// Driver Code``int` `main()``{``    ``string S1 = ``"abc"``;``    ``string S2 = ``"paxzk"``;` `    ``cout << minimumChar(S1, S2);` `    ``return` `0;``}`

## Java

 `// Java program to find the minimum``// number of characters to be``// replaced in string S2, such``// that S1 is a substring of S2``import` `java.io.*;` `class` `GFG``{` `// Function to find the minimum``// number of characters to be``// replaced in string S2, such``// that S1 is a substring of S2``static` `int` `minimumChar(String S1,``                       ``String S2)``{``    ``// Get the sizes of both strings``    ``int` `n = S1.length();``    ``int` `m = S2.length();` `    ``int` `ans = Integer.MAX_VALUE ;` `    ``// Traverse the string S2``    ``for` `(``int` `i = ``0``; i < m - n + ``1``; i++)``    ``{``        ``int` `minRemovedChar = ``0``;` `        ``// From every index in S2, check``        ``// the number of mis-matching``        ``// characters in substring of``        ``// length of S1``        ``for` `(``int` `j = ``0``; j < n; j++)``        ``{``            ``if` `(S1.charAt(j) != S2.charAt(i + j))``            ``{``                ``minRemovedChar++;``            ``}``        ``}` `        ``// Take minimum of prev and``        ``// current mis-match``        ``ans = Math.min(minRemovedChar, ans);``    ``}` `    ``// return answer``    ``return` `ans;``}` `// Driver Code``public` `static` `void` `main (String[] args)``{``    ``String S1 = ``"abc"``;``    ``String S2 = ``"paxzk"``;``    ` `    ``System.out.println(minimumChar(S1, S2));``}``}` `// This code is contributed by Shashank`

## Python3

 `# Python3 program to find the minimum``# number of characters to be replaced``# in string S2, such that S1 is a``# substring of S2``import` `sys` `# Function to find the minimum number of``# characters to be replaced in string S2,``# such that S1 is a substring of S2``def` `minimumChar(S1, S2):``    ` `    ``# Get the sizes of both strings``    ``n, m ``=` `len``(S1), ``len``(S2)` `    ``ans ``=` `sys.maxsize` `    ``# Traverse the string S2``    ``for` `i ``in` `range``(m ``-` `n ``+` `1``):``        ``minRemovedChar ``=` `0` `        ``# From every index in S2, check the``        ``# number of mis-matching characters``        ``# in substring of length of S1``        ``for` `j ``in` `range``(n):``            ``if` `(S1[j] !``=` `S2[i ``+` `j]):``                ``minRemovedChar ``+``=` `1``                ` `        ``# Take minimum of prev and``        ``# current mis-match``        ``ans ``=` `min``(minRemovedChar, ans)``        ` `    ``# return answer``    ``return` `ans``    ` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``    ``S1 ``=` `"abc"``    ``S2 ``=` `"paxzk"``    ``print``(minimumChar(S1, S2))` `# This code is contributed``# by PrinciRaj1992`

## C#

 `// C# program to find the minimum``// number of characters to be``// replaced in string S2, such``// that S1 is a substring of S2``using` `System;` `class` `GFG``{` `// Function to find the minimum``// number of characters to be``// replaced in string S2, such``// that S1 is a substring of S2``static` `int` `minimumChar(String S1,``                       ``String S2)``{``    ``// Get the sizes of both strings``    ``int` `n = S1.Length;``    ``int` `m = S2.Length;` `    ``int` `ans = Int32.MaxValue ;` `    ``// Traverse the string S2``    ``for` `(``int` `i = 0; i < m - n + 1; i++)``    ``{``        ``int` `minRemovedChar = 0;` `        ``// From every index in S2, check``        ``// the number of mis-matching``        ``// characters in substring of``        ``// length of S1``        ``for` `(``int` `j = 0; j < n; j++)``        ``{``            ``if` `(S1[j] != S2[i + j])``            ``{``                ``minRemovedChar++;``            ``}``        ``}` `        ``// Take minimum of prev and``        ``// current mis-match``        ``ans = Math.Min(minRemovedChar, ans);``    ``}` `    ``// return answer``    ``return` `ans;``}` `// Driver Code``public` `static` `void` `Main()``{``    ``String S1 = ``"abc"``;``    ``String S2 = ``"paxzk"``;``    ` `    ``Console.WriteLine(minimumChar(S1, S2));``}``}` `// This code is contributed``// by Akanksha Rai(Abby_akku)`

## PHP

 ``

## Javascript

 ``
Output:

`2`

Time Complexity: O(N * M).

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