> For the complete documentation index, see [llms.txt](https://hao-fu-1.gitbook.io/oj/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hao-fu-1.gitbook.io/oj/string/read-n-characters-given-read4/read-n-characters-given-read4-ii-call-multiple-times.md).

# Read N Characters Given Read4 II - Call multiple times

https\://leetcode.com/problems/read-n-characters-given-read4-ii-call-multiple-times/description/ https\://www\.lintcode.com/problem/read-n-characters-given-read4-ii-call-multiple-times/description

> 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 may be called multiple times.

## Thoughts

给定read4(char \*buf)能从内存中读4个字符给buf，每次调用都会从上次的结束位置往后继续读4个，当内存剩下的未读不足4个时返回本次实际读了多少个；让用read4每次能读n个字符实现read(char \*buf, int n)。read4会从头覆盖buf，因此需要额外空间为4的inbuf存从read4中拿到的字符，然后把它赋值给buf。global变量write\_pos记录inbuf中写入到的位置，read\_pos记录inbuf读到的位置，当read\_pos到达write时说明in\_buf内容都已读完，重新调用read4写入inbuf并重置read\_pos。

## Code

```cpp
// Forward declaration of the read4 API.
int read4(char *buf);

class Solution {
public:
    /**
     * @param buf destination buffer
     * @param n maximum number of characters to read
     * @return the number of characters read
     */
    char inbuf[4];
    int read_pos = 0, write_pos = 0;
    int read(char *buf, int n) {
        for (int i = 0; i < n; ++i) {
            if (read_pos == write_pos) {
                write_pos = read4(inbuf);
                read_pos = 0;
                if (write_pos == 0) return i;
            }
            buf[i] = inbuf[read_pos++];
        }
        return n;
    }
};
```

## Analysis

时间复杂度O(N).