# 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 `read4`reads 4 consecutive characters from the file, then writes those characters into the buffer array`buf`.

The return value is the number of actual characters read.

Note that `read4()`has its own file pointer, much like`FILE *fp`in 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 how`read4`works:

```
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 the`read4`method, implement the method `read`that 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 `read4`but **not** for `read`.
2. The`read`function 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

```java
/* 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;
            }
        }
    }
}
```
