LeetCode/cs/string-compression.cs

public class Solution {
    private static int WriteChar(char[] chars, int start, char c, int count) {
        chars[start++] = c;
        if (count == 1) {
            return start;
        }

        int digits = 1 + (int)Math.Log10(count);

        start += digits - 1;
        for (int offset = 0; offset < digits; count /= 10, ++offset) {
            chars[start - offset] = (char)('0' + (count % 10));
        }

        return start + 1;
    }

    public int Compress(char[] chars) {
        int i = 0;

        var count = 1;
        for (int j = 1; j < chars.Length; ++j) {
            if (chars[j] != chars[j - 1]) {
                i = WriteChar(chars, i, chars[j - 1], count);
                count = 0;
            }

            ++count;
        }

        return WriteChar(chars, i, chars.Last(), count);
    }
}