Read N Characters Given Read4 II - Call multiple times

The API: int read4(char *buf) reads 4 characters at a time from a file.

The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file.

By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.

Note:
The read function may be called multiple times.

思路：

第一次调用时，如果read4读出的多余字符我们要先将其暂存起来，这样第二次调用时先读取这些暂存的字符

第二次调用时，如果连暂存字符都没读完，那么这些暂存字符还得留给第三次调用时使用

这些字符满足先进先出，所以我们可以用一个Queue暂存这些字符.

需要用Queue保存之前多读的character。每次读时，先看Queue里的够不够，如果不够，先读到够为止。

/**
 * The read4 API is defined in the parent class Reader4.
 *     int read4(char[] buf); 
 */

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    private Queue queue = new LinkedList();
    private char[] localbuf = new char[4];
    public int read(char[] buf, int n) {
        if(buf == null || n == 0) {
            return 0;
        }
        
        boolean endOfFile = false;
        // queue.size() < n, 继续读，读满n，或者读到头为止；
        while(queue.size() < n && !endOfFile) {
            int size = read4(localbuf);
            
            if(size < 4) {
                endOfFile = true;
            }
            for(int i = 0; i < size; i++) {
                queue.offer(localbuf[i]);
            }
        }
        // 如果queue里面有并且>= n 或者 < n，就先用queue里面的;
        int len = Math.min(queue.size() , n);
        for(int i = 0; i < len; i++) {
            buf[i] = queue.poll();
        }
        return len;
    }
}