Read N Characters Given Read4 II - Call multiple times

Given a file and assume that you can only read the file using a given method read4, implement a method to read_n_characters.

Method read4:

The API read4reads 4 consecutive characters from the file, then writes those characters into the buffer arraybuf.

The return value is the number of actual characters read.

Note that read4()has its own file pointer, much likeFILE *fpin C.

Definition of read4:

    Parameter:  char[] buf
    Returns:    int

Note: buf[] is destination not source, the results from read4 will be copied to buf[]

Below is a high level example of howread4works:

File file("abcdefghijk"); // File is "abcdefghijk", initially file pointer (fp) points to 'a'
char[] buf = new char[4]; // Create buffer with enough space to store characters
read4(buf); // read4 returns 4. Now buf = "abcd", fp points to 'e'
read4(buf); // read4 returns 4. Now buf = "efgh", fp points to 'i'
read4(buf); // read4 returns 3. Now buf = "ijk", fp points to end of file

Method read:

By using theread4method, implement the method readthat readsncharacters from the file and store it in the buffer array buf. Consider that you cannot manipulate the file directly.

The return value is the number of actual characters read.

Definition of read:

    Parameters:    char[] buf, int n
    Returns:       int

Note: buf[] is destination not source, you will need to write the results to buf[]

Example

Example 1:

Input: 
file = "abc", n = 4

Output: 
3

Explanation:
After calling your read method, buf should contain "abc". 
We read a total of 3 characters from the file, so return 3. 
Note that "abc" is the file's content, not buf. 
buf is the destination buffer that you will have to write the results to.

Example 2:

Input: 
file = "abcde", n = 5

Output: 
5

Explanation: 
After calling your read method, buf should contain "abcde". 
We read a total of 5 characters from the file, so return 5.

Example 3:

Input: 
file = "abcdABCD1234", n = 12

Output: 
12

Explanation: 
After calling your read method, buf should contain "abcdABCD1234". 
We read a total of 12 characters from the file, so return 12.

Example 4:

Input: 
file = "leetcode", n = 5

Output: 
5

Explanation: 
After calling your read method, buf should contain "leetc". 
we read a total of 5 characters from the file, so return 5.

Note:

1. Consider that you cannot manipulate the file directly, the file is only accesible for read4but not for read.

2. Thereadfunction will only be called once for each test case.

3. You may assume the destination buffer array, buf, is guaranteed to have enough space for storing n characters.

Note

n是需要读的字符数目，buf是输入的字符串（以char的形式）

利用一个4长度的buf4数组进行缓存，记录offset，不停循环写入

判断条件是i < size && offset < n

i是buf4的头指针，size是read4(buf4)的返回值

offset遍历输入，必须小于n且i要小于size（example1）

Code

/* 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[] buf4 = new char[4];
        int offset = 0;

        while (true) {
            int size = read4(buf4);
            for (int i = 0; i < size && offset < n; i++) {
                buf[offset++] = buf4[i];
            }
            if (size == 0 || offset == n) {
                return offset;
            }
        }
    }
}