Problem Description

You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times.

Return the length of the longest substring containing the same letter you can get after performing the above operations.

Example 1:

  • Input: s = "ABAB", k = 2
  • Output: 4
  • Explanation: Replace the two 'A's with two 'B's or vice versa.

Example 2:

  • Input: s = "AABABBA", k = 1
  • Output: 4
  • Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA". The substring "BBBB" has the longest repeating letters, which is 4. There may exists other ways to achieve this answer too.

My Idea

The idea here was to make use of the Sliding Window technique and keep a hashmap for the frequency of the characters in the window. For every char we check if we can make a string using k replacement, if not we move the window from the left until possible.

My solution

# Time Complexity: O(n)
def characterReplacement(s:str, k:int) -> int:
    l = 0
    res = 0
    freq = dict()
    for r in range(0,len(s)):
        if s[r] in freq:
            freq[s[r]]+=1
        else:
            freq[s[r]]=1
        while (r-l)+1-max(freq.values()) > k:
            freq[s[l]]-=1
            l+=1
        res = max(res, (r-l)+1)
    return res