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)

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.

- 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’).
- For doing this we traverse whole string and whenever we find a different character, we increase the count.
- 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.
- When count will be equal to k (at left index) then at that point the length will be rightIndex-leftIndex+1.
- We repeat this process until we reach at the end of string and at that point we will return the maximum length.
- 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

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