
Problem Statement
In the given scenario, a teacher is preparing a test comprising n true/false questions represented by the characters 'T' for true and 'F' for false. The objective is to manipulate the sequence of answers by a specified number of operations to maximize the length of consecutive identical answers, which could either be all 'T's or all 'F's. The allowable operation, which can be performed a maximum of k times, involves changing any question's answer from 'T' to 'F' or vice versa. The challenge is to determine the maximum number of consecutive identical answers that can be achieved by utilizing up to k operations.
Examples
Example 1
Input:
answerKey = "TTFF", k = 2
Output:
4
Explanation:
We can replace both the 'F's with 'T's to make answerKey = "TTTT". There are four consecutive 'T's.
Example 2
Input:
answerKey = "TFFT", k = 1
Output:
3
Explanation:
We can replace the first 'T' with an 'F' to make answerKey = "FFFT". Alternatively, we can replace the second 'T' with an 'F' to make answerKey = "TFFF". In both cases, there are three consecutive 'F's.
Example 3
Input:
answerKey = "TTFTTFTT", k = 1
Output:
5
Explanation:
We can replace the first 'F' to make answerKey = "TTTTTFTT" Alternatively, we can replace the second 'F' to make answerKey = "TTFTTTTT". In both cases, there are five consecutive 'T's.
Constraints
n == answerKey.length1 <= n <= 5 * 104answerKey[i]is either'T'or'F'1 <= k <= n
Approach and Intuition
The goal is to maximize contiguous segments of either 'T's or 'F's. Considering the constraints that operations can be used up to k times, we can use a sliding window technique with two pointers to solve this problem efficiently. Here's how we can break down the process:
- Use two pointers,
leftandright, to represent the current window of consecutive answers within theanswerKey. - Expand the window by moving the
rightpointer and counting the number of either'T'or'F'within the window. - If the count of characters that need to be changed exceeds
k, adjust theleftpointer to shrink the window until the count is at or belowk. - The length of the window (i.e.,
right - left + 1) will give the maximum consecutive sequence for the current character focus. - Repeat this process twice - once focusing on maximizing
'T's and once on'F's. - The larger of the two results from the above iterations will be the maximum number of consecutive identical answers that can be achieved.
From the examples provided:
- For Example 1, changing both 'F's to 'T's leads to "TTTT", achieving 4 consecutive 'T's with 2 changes.
- For Example 2, changing one 'T' to 'F' either way maximizes consecutive 'F's to 3, demonstrating the flexibility in selecting which character to optimize.
- In Example 3, changing one 'F' at different positions can optimize up to 5 consecutive 'T's, illustrating how strategic changes in larger sequences can yield significant stretches of identical answers.
By using this sliding window technique, we efficiently explore and adjust segments of the answerKey to find the optimal length of consecutive answers, ensuring we do not exceed the allowed k changes.
Solutions
- C++
- Java
- Python
class Solution {
public:
int getMaxAnswerStreak(string answerKey, int k) {
int maxStreak = 0;
unordered_map<char, int> countLetters;
for (int r = 0; r < answerKey.size(); r++) {
countLetters[answerKey[r]]++;
int smallest = min(countLetters['T'], countLetters['F']);
if (smallest <= k) {
maxStreak++;
} else {
countLetters[answerKey[r - maxStreak]]--;
}
}
return maxStreak;
}
};
The problem "Maximize the Confusion of an Exam" is efficiently tackled using a C++ implementation that employs the sliding window technique alongside a frequency counting strategy. Here’s a breakdown of the solution:
- Initialize
maxStreakto zero to keep track of the maximum number of consecutive answers that match a certain criterion. - Utilize an
unordered_mapnamedcountLettersto monitor the frequency of each character ('T' for true and 'F' for false) within the current sliding window. - Traverse the
answerKeyusing a loop, dynamically adjusting the sliding window:- Add the current character to
countLetters. - Determine the count of the less frequent answer within the window using the
minfunction on the values fromcountLetters. - If the counts of the less frequent answer is less than or equal to
k, increasemaxStreakto include the current character. - If not, modify the window by decrementing the count of the character that falls out of the left side of the current window length to maintain the allowed number of transformations denoted by
k.
- Add the current character to
- Continue this process until you've evaluated all possible substrings of
answerKey. - Return
maxStreak, which now holds the maximum length of a contiguous segment of answers where one can change up tokanswers to either all 'T' or all 'F', thereby maximizing the confusion or difficulty of the exam.
This approach ensures a swift traversal through the string while dynamically managing and evaluating the possibilities of maximizing confusion under the stated constraints.
class Solution {
public int longestSequence(String responses, int k) {
int lengthMax = 0;
Map<Character, Integer> frequency = new HashMap<>();
for (int end = 0; end < responses.length(); end++) {
frequency.put(responses.charAt(end), frequency.getOrDefault(responses.charAt(end), 0) + 1);
int minimum = Math.min(frequency.getOrDefault('T', 0), frequency.getOrDefault('F', 0));
if (minimum <= k) {
lengthMax++;
} else {
frequency.put(responses.charAt(end - lengthMax), frequency.get(responses.charAt(end - lengthMax)) - 1);
}
}
return lengthMax;
}
}
The provided Java solution focuses on finding the maximum length of a substring in a string where you can make at most k substitutions to convert it either entirely to 'T's or entirely to 'F's. The function longestSequence uses a sliding window approach with a hashmap to maintain the frequency of each character within the window.
Here’s a breakdown of how the solution operates:
- Initialize
lengthMaxto 0 to store the maximum length of the substring that meets the condition. - Create a
frequencyhashmap to track the number of occurrences of each character in the current sliding window. - Iterate through each character in the
responsesstring using an indexend. - Update the frequency of the current character in the hashmap.
- Calculate
minimum, the smaller frequency between 'T' and 'F' within the current window. - If
minimumis less than or equal tok, the substring can be made uniform by replacing theseminimumcharacters, and thus, increaselengthMax. - If not, adjust the window by reducing the frequency of the character at the start of the window (
end - lengthMax) and continue. - Finally, the function returns
lengthMax, which represents the length of the longest valid substring found.
This effectively tracks the longest stretch where you can replace up to k characters to make the substring uniform, using an optimal approach through a sliding window mechanism combined with frequent character tracking.
class Solution:
def maximumConsecutiveAnswers(self, answers: str, maxFlips: int) -> int:
longest_sequence = 0
char_freq = collections.Counter()
for end in range(len(answers)):
char_freq[answers[end]] += 1
least_frequent = min(char_freq['T'], char_freq['F'])
if least_frequent <= maxFlips:
longest_sequence += 1
else:
char_freq[answers[end - longest_sequence]] -= 1
return longest_sequence
The provided Python3 code defines a function maximumConsecutiveAnswers that determines the maximum number of consecutive 'T' or 'F' answers you can obtain in a string answers by flipping at most maxFlips characters. Here is a summary of how this function works:
- Initialize
longest_sequenceto store the length of the longest sequence achievable. - Use a
Counterfrom thecollectionsmodule to track the frequency of 'T' and 'F' answers. - Iterate through each character in the
answersstring using a loop. - Update the frequency of the currently processed character.
- Calculate the frequency of the least frequent character between 'T' and 'F'.
- If the number of least frequent characters is less than or equal to
maxFlips, incrementlongest_sequence. - If not, decrease the count of the character at the start of the current longest sequence to adjust the window size.
- Finally,
longest_sequenceis returned, which gives the maximum number of consecutive characters that can be achieved by flipping at mostmaxFlipscharacters.
Adapt this approach by modifying variables and conditions to suit specific needs or different variations of the problem.