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.
Key Insights
- The problem can be solved using a sliding window approach.
- We maintain a count of the characters in the current window.
- The window should be expanded until the number of characters that need to be replaced exceeds
k. - We track the maximum frequency of any character in the current window to determine if we can convert the rest to that character.
- The difference between the window size and the maximum frequency gives the number of replacements needed.
Space and Time Complexity
Time Complexity: O(n)
Space Complexity: O(1) (for the frequency count of 26 uppercase letters)
Solution
The solution uses a sliding window technique where we maintain two pointers to define the current window within the string. We use a frequency array to count the occurrences of each character in the window. The algorithm expands the window by moving the right pointer and calculates the number of characters that need to be replaced. If the number of replacements exceeds k, we shrink the window from the left. The largest valid window size found during the process is the answer.