Read N Characters Given Read4 I/II

要点：这题的要点就是搞清楚几个变量的内在逻辑：只有buffer是整4 bytes的。而client要读的bytes（需求）和实际上disk上有的bytes（供给）都是不整的。所以，

• 循环的条件就是either 供给不足 or 需求不足：供给不足的判定是上一轮数据不够4 bytes的mark，而需求不足是toRead的计数<=0
• 所以循环体内，就是min(读进来的bytes, toRead)来把数据copy到buffer里，同时更新toRead和结束条件
• II就是加了个buffer来存上一轮的leftover和offset，在下一次读的时候，把剩余数据假装作为一个read4来处理。
  • 为什么要用bufSize而不是offset本身？offset表示的是数据的起始位置（当前位置是还没读的），可能有数据，也可能没有。
  • 所以逻辑是只有bufSize==0才读4 bytes，global buffer做缓冲区，而bufSize永远标示待读区间的大小
  • offset不断从0向右，然后再回到0：4 bytes肯定一次读进来

I: https://repl.it/Cjjw/1
II: https://repl.it/CjkR/2
错误点：

• self.offset>=4，不是>4
• 别忘了eof（在bufsize==0里面）

# 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 will only be called once for each test case.# Hide Company Tags Facebook
# Hide Tags String
# Hide Similar Problems (H) Read N Characters Given Read4 II - Call multiple times# 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)"""eof, nbytes = False, 0while not eof and nbytes<n:buf4 = [""]*4nread = read4(buf4)if nread<4:eof = Truennext = min(nread, n-nbytes)for i in xrange(nnext):buf[nbytes+i]=buf4[i]nbytes+=nnextreturn nbytes

# 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.# Hide Company Tags Bloomberg Google Facebook
# Hide Tags String
# Hide Similar Problems (E) Read N Characters Given Read4# The read4 API is already defined for you.
# @param buf, a list of characters
# @return an integer
# def read4(buf):class Solution(object):def __init__(self):self.bufbytes, self.offset = 0,0self.buf4 = [""]*4def 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)"""eof, nbytes = False, 0while not eof and nbytes<n:if self.bufbytes==0:self.bufbytes = read4(self.buf4)if self.bufbytes<4: # error: don't forgeteof = Truetoread = min(n-nbytes, self.bufbytes)#print toread,self.offsetfor i in xrange(toread):buf[nbytes+i]=self.buf4[self.offset+i]self.offset+=toreadif self.offset >= 4: # error: >=4, last index is 3self.offset-=4self.bufbytes-=toreadnbytes+=toreadreturn nbytes