int read4(char *buf);

class Solution
{
    queue<char> q; //一次读取4个，可能多了，存储起来
public:
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    int read(char *buf, int n)
    {
        if (!buf || n <= 0)
            return 0;
        int len = 0, total = 0;
        int offset = 0;
        while (!q.empty()) //临时存储区还有先用掉
        {
            buf[offset++] = q.front();
            q.pop();
            total++;
            if (offset == n) //个数到了
                return n;
        }
        do
        {
            len = read4(buf + offset); //读取了len个有效字符，写入offset位置
            total += len;
            offset += len;
        } while (len == 4 && len < n);

        if (total > n)
        { //读取多了，存到临时存储区
            for (int i = n; i < total; i++)
                q.push(buf[i]);
        }
        return min(n, total);
    }
};