[LeetCode] String Compression

Problem

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 {
    public int compress(char[] chars) {
        int i = 0, index = 0;
        while (i < chars.length) {
            char ch = chars[i];
            int count = 0;
            while (i < chars.length && chars[i] == ch) {
                i++;
                count++;
            }
            chars[index++] = ch;
            if (count != 1) {
                String str = Integer.toString(count);
                for (char digit: str.toCharArray()) {
                    chars[index++] = digit;
                }
            }
        }
        return index;
    }
}