String Compression

You are 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. For example, 10 is represented as ["1","0"].

Let k be the length of the compressed string s. You must modify the first k characters of chars array and return k.

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

Example 1:

Input: chars = ["a","a","a","a","a","a","a","a","a","a","a"]

Output: 3

Explanation: The compressed string is "a11" and the first 3 characters of the input array should be ["a","1","2"].

Example 2:

Input: chars = ["A"]

Output: 1

Explanation: The compressed string is "A" and the first 1 characters of the input array should be ["A"].

Example 3:

Input: chars = ["1","1","2"]

Output: 3

Explanation: The compressed string is "122" and the first 3 characters of the input array should be ["1","2","2"].

Constraints:

• 1 <= chars.length <= 2000
• chars[i] is a lowercase English letter, uppercase English letter, digit, or symbol.

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