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 will only be called once for each test case.

说明:1. read4(char * buf)的这个buf起初是空的,然后read4读出来的东西存在这个buf里;

     2. read函数的buf是destination, 是我们需要往里面依次写通过read4读出来的东西

 

首先read4是一个读文件的函数,只能读4个char。
char [] buffer = new char[4]
int size = read4(buffer)
这里read4将空buffer改成了file的 4 个char.
然后关于parameter的定义是:

* @param buf Destination buffer,我后来理解这个应该是个足够大的buffer,需要将读到的file放到这个buffer里面去
* @param n Maximum number of characters to read,想要读n个char,但是有可能读不到那么多,因为有可能n比文件的size大
* @return The number of characters read,实际读到的char数,小于等于n

由于不知道文件的大小,所以只能一次一次用4个char的buffer用read4去读,存在以下两种结束方式(假设文件大小为size):

1. n >> size, 比如n = 50, size = 23, 最后一次读的大小为<4, 下一次就不读了,然后把这一次读的char数oneRead赋给目标buf

至于n = 50, size = 20, 最后一次读到0个,不会进行赋值,下一次也不读了

2. n << size,比如n = 23, size = 50, read4每次会读满4个,但是我们只取3个,取零头的方法是int actRead = Math.min(n-haveRead, oneRead); 下一次OneRead+haveRead>n, 也就停止了

想清楚了这些,就可以写了

 1 /* The read4 API is defined in the parent class Reader4.
 2       int read4(char[] buf); */
 3 
 4 public class Solution extends Reader4 {
 5     /**
 6      * @param buf Destination buffer
 7      * @param n   Maximum number of characters to read
 8      * @return    The number of characters read
 9      */
10     public int read(char[] buf, int n) {
11        char[] buffer = new char[4];
12        int haveRead = 0;
13        boolean lessthan4 = false;
14        
15        while (!lessthan4 && haveRead < n) {
16            int oneRead = read4(buffer);
17            if (oneRead < 4) lessthan4 = true;
18            int actRead = Math.min(n-haveRead, oneRead);
19            while (int i=0; i<actRead; i++) {
20                buf[haveRead+i] = buffer[i];
21            }
22            haveRead += actRead;
23        }
24        return haveRead;
25     }
26 }