157. Read N Characters Given Read4-优快云博客

本文介绍了一种利用已有的read4 API实现自定义read函数的方法。通过继承Reader4类并覆盖read方法，实现了从文件中按需读取指定数量字符的功能。特别注意到了文件末尾读取不足四个字符的情况。

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题目：
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.

解答：
读懂题目很重要，还是要多写写这种实际的api的问题。

/* 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) {
        boolean end = false;
        int total = 0;
        char[] temp = new char[4];
        //有两种情况：1. 需要read的character个数是n，但实际文件中character个数小于n个，所以我们只能返回实际character的个数；2. 需要read的character个数n小于实际文件中的个数，所以最后一步只需要n - total个character就可以。
        while (!end && total < n) {
            int count = read4(temp);
            //实际文件读到头了的情况
            end = count < 4;
            //需要读的文件个数达到了的情况
            count = Math.min(count, n - total);
            for (int i = 0; i < count; i++) {
                buf[total++] = temp[i];
            }
        }
        return total;
    }
}