String Compression

Problem

Given an array of characters chars, compress it using the following algorithm:

Begin with an empty string s. For each group of consecutive repeating characters in chars:

• If the group's length is 1, append the character to s.
• Otherwise, append the character followed by the group's length.

The compressed string s should not be returned separately, but instead, be stored in the input character array chars. Note that group lengths that are 10 or longer will be split into multiple characters in chars.

After you are done modifying the input array, return the new length of the array.

You must write an algorithm that uses only constant extra space.

Examples

Example 1:

Input:
chars = ["a","a","b","b","c","c","c"]
Output:
 Return 6, and the first 6 characters of the input array should be: ["a","2","b","2","c","3"]
Explanation: The groups are "aa", "bb", and "ccc". This compresses to "a2b2c3".

Example 2:

Input:
chars = ["a"]
Output:
 Return 1, and the first character of the input array should be: ["a"]
Explanation: The only group is "a", which remains uncompressed since it's a single character.

Example 3:

Input:
chars = ["a","b","b","b","b","b","b","b","b","b","b","b","b"]
Output:
 Return 4, and the first 4 characters of the input array should be: ["a","b","1","2"].
Explanation: The groups are "a" and "bbbbbbbbbbbb". This compresses to "ab12".

Solution

Method 1 - Iterative Greedy Method

Code

public int compress(char[] chars) {
	int n = chars.length;
	if(n <= 1){
		return n;
	}
	int i = 0;
	int idx = 0;
	while (i < n){
		int j = i;
		while (j < n && chars[j] == chars[i]){
			j++;
		}
		chars[idx++] = chars[i];
		// the substring with repeating chars should have size strictly greater than 1
		if(j - i > 1){
			 String count = Integer.toString(j - i);
			 for (char ch: count.toCharArray()){
				 chars[idx++] = ch;
			 }
		}

		// update i
		i = j;
	}
	return idx;
}

Complexity

• Time: O(N) where N is length of char array
• Space: O(1)

Method 2 - Using RLE Codec

This is very similar to [Run-length Encoding](run-length-encoding). We can join the characters, use the codec.encode and then get the length of the codec.

Code

Java

public int compress(char[] chars) {
	return RLECodec.encode(new String(chars)).length();
}