3163. String Compression III
Difficulty: Medium
Topics: String
Given a string word, compress it using the following algorithm:
- Begin with an empty string comp. While word is not empty, use the following operation:
- Remove a maximum length prefix of word made of a single character c repeating at most 9 times.
- Append the length of the prefix followed by c to comp.
Return the string comp.
Example 1:
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Input: word = "abcde"
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Output: "1a1b1c1d1e"
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Explanation: Initially, comp = "". Apply the operation 5 times, choosing "a", "b", "c", "d", and "e" as the prefix in each operation.
- For each prefix, append "1" followed by the character to comp.
Example 2:
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Input: word = "aaaaaaaaaaaaaabb"
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Output: "9a5a2b"
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Explanation: Initially, comp = "". Apply the operation 3 times, choosing "aaaaaaaaa", "aaaaa", and "bb" as the prefix in each operation.
- For prefix "aaaaaaaaa", append "9" followed by "a" to comp.
- For prefix "aaaaa", append "5" followed by "a" to comp.
- For prefix "bb", append "2" followed by "b" to comp.
Constraints:
- 1 <= word.length <= 2 * 105
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word consists only of lowercase English letters.
Hint:
- Each time, just cut the same character in prefix up to at max 9 times. It’s always better to cut a bigger prefix.
Solution:
We can use a greedy approach to compress the string by taking the longest possible prefix of repeating characters (up to 9 occurrences at a time) and then appending the length of the prefix along with the character to the result.
Here's the step-by-step solution:
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Initialize Variables:
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comp (the compressed string) starts as an empty string.
- Use a pointer or index i to track the position in the word.
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Loop through word:
- While there are characters left in word, find the longest prefix of repeating characters that does not exceed 9 characters.
- Count how many times the current character repeats consecutively, up to a maximum of 9.
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Append to Compressed String:
- Append the count followed by the character to comp.
- Move the pointer i forward by the number of characters processed.
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Return Result:
- After processing the entire string, return the compressed string comp.
Let's implement this solution in PHP: 3163. String Compression III
Explanation:
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Counting Loop: We use a while loop inside the main loop to count consecutive characters (up to 9) for each unique character in word.
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Appending Results: After counting each sequence, we append the count and character directly to comp.
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Pointer Update: The main loop pointer i moves forward by the number of characters counted, effectively reducing the size of the remaining string in each iteration.
Complexity Analysis
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Time Complexity: O(n), where n is the length of word. Each character is processed once.
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Space Complexity: O(n) for the compressed result stored in comp.
This solution is efficient and handles edge cases, such as sequences shorter than 9 characters or a single occurrence of each character.
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