443. String Compression

class Solution {
    public int compress(char[] chars) {
        int i = 0, j = 0;
        while (j < chars.length) {
            // 统计当前元素次数并记录
            char c = chars[j];
            chars[i++] = c;
            int count = 0;
            while (j < chars.length && chars[j] == c) {
                j++;
                count++;
            }
            // now: j == chars.length or chars[j] != c

            if (count > 1) {
                int endIndex = i;
                while (count != 0) {
                    chars[endIndex++] = (char) ('0' + count % 10);
                    count /= 10;
                }
                // now: endIndex point to next char
                reverse(chars, i, endIndex - 1);
                i = endIndex;
            }
        }

        return i;
    }

    private void reverse(char[] chars, int i, int j) {
        while (i < j) {
            swap(chars, i++, j--);
        }
    }

    private void swap(char[] chars, int i, int j) {
        char tmp = chars[i];
        chars[i] = chars[j];
        chars[j] = tmp;
    }
}

References

443. String Compression