157.Read N Characters Given Read4

Tags: [file], [file_system], [queue]

Com: {fb}

Link: https://leetcode.com/problems/read-n-characters-given-read4/\#/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 theread4API, implement the functionint read(char *buf, int n)that readsncharacters from the file.

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

Solution: Queue

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

from collections import deque

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)
        """
        if not n:
            return 0

        queue = deque()
        while len(queue) < n:
            sub_buf = ['' for _ in xrange(4)]
            num_of_chars = read4(sub_buf)
            if not num_of_chars:
                break
            for i in xrange(num_of_chars):
                queue.append(sub_buf[i])

        index = 0
        while index < n and queue:
            buf[index] = queue.popleft()
            index += 1

        return index

Revelation:

• Do not forget to check whether num__of__chars is 0 during the first while loop.

Another 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)
        """
        if not n:
            return 0

        index = 0
        while n > 4:
            sub_buf = ['' for _ in xrange(4)]
            num_of_chars = read4(sub_buf)
            if not num_of_chars:
                break

            for i in xrange(num_of_chars):
                buf[index] = sub_buf[i]
                index += 1
            n -= num_of_chars

        if n:
            sub_buf = ['' for _ in xrange(4)]
            num_of_chars = read4(sub_buf)
            for i in xrange(min(n, num_of_chars)):
                buf[index] = sub_buf[i]
                index += 1

        return index