443. String Compression

https://leetcode.com/problems/string-compression/description/

Discription

Given an array of characters, compress it in-place.

The length after compression must always be smaller than or equal to the original array.

Every element of the array should be a character (not int) of length 1.

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

Follow up:
Could you solve it using only O(1) extra space?

Example 1:

Input:
["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:
"aa" is replaced by "a2". "bb" is replaced by "b2". "ccc" is replaced by "c3".

Example 2:

Input:
["a"]
Output:
Return 1, and the first 1 characters of the input array should be: ["a"]
Explanation:
Nothing is replaced.

Example 3:

Input:
["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:
Since the character "a" does not repeat, it is not compressed. "bbbbbbbbbbbb" is replaced by "b12".
Notice each digit has it's own entry in the array.

Note:

All characters have an ASCII value in [35, 126].
1 <= len(chars) <= 1000.

Solution

class Solution:
    def compress(self, chars):
        """
        :type chars: List[str]
        :rtype: int
        """
        length = len(chars)
        if length==0:
            return 0
        count = 1
        i = 1
        modify_pt = 0
        while i <= length:
            if i==length or chars[i]!=chars[i-1]:
                chars[modify_pt] = chars[i-1]
                modify_pt += 1
                if count>1:
                    for s in str(count):
                        chars[modify_pt] = s
                        modify_pt += 1
                    count = 1
                i += 1
            else:
                count += 1
                i += 1
        return modify_pt