443. String Compression

/*
Time Complexity: O(N)
Space Complexity: O(1)

N = length of chars array.
*/
class Solution {
    public int compress(char[] chars) {
        if (chars == null || chars.length == 0) {
            return 0;
        }
        if (chars.length == 1) {
            return 1;
        }
        int index = 0;
        int ansIndex = 0;

        while(index < chars.length) {
            char curChar = chars[index];
            int count = 0;
            while (index < chars.length && chars[index] == curChar) {
                count++;
                index++;
            }
            chars[ansIndex++] = curChar;
            if (count > 1) {
                for (char c : Integer.toString(count).toCharArray()) {
                    chars[ansIndex++] = c;
                }
            }
        }
        return ansIndex;
    }
}