# Maximum length substring having all same characters after k changes

We have a string of length n, which consist only UPPER and LOWER CASE characters and we have a number k (always less than n and greater than 0). We can make at most k changes in our string such that we can get a sub-string of maximum length which have all same characters.

Examples:

```n = length of string
k = changes you can make
Input : n = 5 k = 2
str = ABABA
Output : maximum length = 5
which will be (AAAAA)

Input : n = 6 k = 4
str = HHHHHH
Output : maximum length=6
which will be(HHHHHH)
```

## Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

We check for each character of English alphabet (both upper and lower cases one by one). We are basically looking for maximum length of sub-string that can be formed by each character and whichever character will form the sub-string of maximum length then that length will be our answer.

1. We check for maximum length of sub-string that can be formed by every character in a set of 52 characters (From ‘A’ to ‘Z’ and from ‘a’ to ‘z’).
2. For doing this we traverse whole string and whenever we find a different character, we increase the count.
3. If count becomes greater than k (at right index), we again start from 0th index and if we found different character we will decrease the count.
4. When count will be equal to k (at left index) then at that point the length will be rightIndex-leftIndex+1.
5. We repeat this process until we reach at the end of string and at that point we will return the maximum length.
6. We do this for all characters and finally return the maximum length.

## C++

```// C++ program to find maximum length equal
// character string with k changes
#include <iostream>
using namespace std;

// function to find the maximum length of
// substring having character ch
int findLen(string& A, int n, int k, char ch)
{
int maxlen = 1;
int cnt = 0;
int l = 0, r = 0;

// traverse the whole string
while (r < n) {

/* if character is not same as ch
increase count */
if (A[r] != ch)
++cnt;

/* While  count > k  traverse the string
again until count becomes less than k
and decrease the count when characters
are not same */
while (cnt > k) {
if (A[l] != ch)
--cnt;
++l;
}

/* length of substring will be rightIndex -
leftIndex + 1. Compare this with the maximum
length and return maximum length */
maxlen = max(maxlen, r - l + 1);
++r;
}
return maxlen;
}

// function which returns maximum length of substring
int answer(string& A, int n, int k)
{
int maxlen = 1;
for (int i = 0; i < 26; ++i) {
maxlen = max(maxlen, findLen(A, n, k, i+'A'));
maxlen = max(maxlen, findLen(A, n, k, i+'a'));
}
return maxlen;
}

// Driver code
int main()
{
int n = 5, k = 2;
string A = "ABABA";
cout << "Maximum length = " << answer(A, n, k) << endl;

n = 6, k = 4;
string B = "HHHHHH";
cout << "Maximum length = " << answer(B, n, k) << endl;
return 0;
}
```

## Java

```// Java program to find maximum length equal
// character string with k changes

public class GFG
{
// method to find the maximum length of
// substring having character ch
static int findLen(String A, int n, int k, char ch)
{
int maxlen = 1;
int cnt = 0;
int l = 0, r = 0;

// traverse the whole string
while (r < n) {

/* if character is not same as ch
increase count */
if (A.charAt(r) != ch)
++cnt;

/* While  count > k  traverse the string
again until count becomes less than k
and decrease the count when characters
are not same */
while (cnt > k) {
if (A.charAt(l) != ch)
--cnt;
++l;
}

/* length of substring will be rightIndex -
leftIndex + 1. Compare this with the maximum
length and return maximum length */
maxlen = Math.max(maxlen, r - l + 1);
++r;
}
return maxlen;
}

// method which returns maximum length of substring
static int answer(String A, int n, int k)
{
int maxlen = 1;
for (int i = 0; i < 26; ++i) {
maxlen = Math.max(maxlen, findLen(A, n, k, (char) (i+'A')));
maxlen = Math.max(maxlen, findLen(A, n, k, (char) (i+'a')));
}
return maxlen;
}

// Driver Method
public static void main(String[] args)
{
int n = 5, k = 2;
String A = "ABABA";
System.out.println("Maximum length = " + answer(A, n, k));

n = 6; k = 4;
String B = "HHHHHH";
System.out.println("Maximum length = " + answer(B, n, k));
}
}
```

Output:

```Maximum length = 5
Maximum length = 6
```

This article is contributed by Niteesh Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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