157. Read-N-Characters-Given-Read4.md

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.

Example 1:

Input: buf = "abc", n = 4
Output: "abc"
Explanation: The actual number of characters read is 3, which is "abc".

Example 2:

Input: buf = "abcde", n = 5
Output: "abcde"

Note: The read function will only be called once for each test case.

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又是一道垃圾题目，连题干都是错的……

题目中说：

Input: buf = "abc", n = 4
Output: "abc"
Explanation: The actual number of characters read is 3, which is "abc".

完全是骗人的，真实情况是：

• 如果有一个file = "abc"，我们传入一个buf空数组：buf = ["", "", "", ..... ]。

在调用完 read(buf, n)之后(例如n=4)，我们应该输出 output = 3（读到了三个字符）。

而且buf数组应该为：buf = ["a", "b", "c", "", "", "", …….]

• 如果调用read(buf, n)时n = 2那么我们应该输出 output = 2（因为最多读取2个字符）。

而且buf数组应该为：buf = ["a", "b", "", "", "", …….]

python solution

# The read4 API is already defined for you.
# @param buf, a list of characters
# @return an integer
# def read4(buf):

class Solution(object):
    def read(self, buf, n):
        """
        :type buf: Destination buffer (List[str])
        :type n: Maximum number of characters to read (int)
        :rtype: The number of characters read (int)
        """
        res = 0
        while n > 0:
            buf4 = [""] * 4
            length = read4(buf4)
            if not length:  # 如果文件中已经读不到字符，直接返回结果
                return res
            for i in range(min(length, n)):  # 如果读取到字符且还没有达到n个，继续填充buf。
                buf[res] = buf4[i]
                res += 1
                n -= 1
        return res