157. Read N Characters Given Read4 - jiejackyzhang/leetcode-note GitHub Wiki

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.

简单讲就是已经有个API - read4(char[] buf)，每次从文件里读取最多4个bytes到char[] buf里。要求根据read4来实现read(char[] buf, n)，就是可以从文件中读取n个字符。

我们首先创建一个read4Buf用来保存每次调用read4 api所返回的字符。建立两个变量，一个read4Count用来记录read4调用实际返回了多少个字符，另外一个totalCharRead表示我们总共已经读取了多少字符。使用一个while循环，在(read4Count ＝ read4(read4Buf)) > 0的时候，每次拷贝这回read4调用读取的字符到输出buf中。注意拷贝时的条件是i < read4Count && totalCharRead < n

/* 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   Maximum number of characters to read
     * @return    The number of characters read
     */
    public int read(char[] buf, int n) {
        char[] read4Buf = new char[4];
        int totalCharRead = 0;
        int read4Count = 0;
        while ((read4Count = read4(read4Buf)) > 0) {
            for (int i = 0; i < read4Count && totalCharRead < n; i++) {
                buf[totalCharRead++] = read4Buf[i];
            }
        }
        return totalCharRead;
    }
}