443 - String Compression.

Medium

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.

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".

Constraints:

• 1 <= chars.length <= 2000

• chars[i] is a lowercase English letter, uppercase English letter, digit, or symbol.