3163. String Compression III

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:

• Input: word = "abcde"
• Output: "1a1b1c1d1e"
• 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:

• Input: word = "aaaaaaaaaaaaaabb"
• Output: "9a5a2b"
• 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
• word consists only of lowercase English letters.

Hint:

1. 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:

1. Initialize Variables:

   • comp (the compressed string) starts as an empty string.
   • Use a pointer or index i to track the position in the word.

2. 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.

3. Append to Compressed String:

   • Append the count followed by the character to comp.
   • Move the pointer i forward by the number of characters processed.

4. Return Result:

   • After processing the entire string, return the compressed string comp.

Let's implement this solution in PHP: 3163. String Compression III

<?php
/**
 * @param String $word
 * @return String
 */
function compressString($word) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Test cases
echo compressString("abcde");          // Output: "1a1b1c1d1e"
echo "\n";
echo compressString("aaaaaaaaaaaaaabb"); // Output: "9a5a2b"
?>

Explanation:

• Counting Loop: We use a while loop inside the main loop to count consecutive characters (up to 9) for each unique character in word.
• Appending Results: After counting each sequence, we append the count and character directly to comp.
• 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

• Time Complexity: O(n), where n is the length of word. Each character is processed once.
• 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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