python实现栈的实例

栈遵循后进先出原文，本文将用python 实现三个用栈的实例 栈在上一篇文章中已用python实现， 下面实例将引用这个栈。
1、实现一个文件中各行的逆置

def reverse_file(filename):
  """Overwrite given file with its contents line-by-line reversed."""
  S = ArrayStack()
  original = open(filename)       
  for line in original:
    S.push(line.rstrip('\n'))     # we will re-insert newlines when writing
  original.close()

  # now we overwrite with contents in LIFO order
  output = open(filename, 'w')    # reopening file overwrites original
  while not S.is_empty():
    output.write(S.pop() + '\n')  # re-insert newline characters
  output.close()

2、实现在算数表达式中分隔符的匹配
算数表达式中有成对的符号，比如：

• 括弧 “(”和“)“
• 中括弧 “[”和“]”
• 花括弧 "{"和"}"
  正确：(){[]}
  错误：([{ ]})

from array_stack import ArrayStack
def is_matched(expr):
  """Return True if all delimiters are properly match; False otherwise."""
  lefty = '({['                               # opening delimiters
  righty = ')}]'                              # respective closing delims
  S = ArrayStack()
  for c in expr:
    if c in lefty:
      S.push(c)                               # push left delimiter on stack
    elif c in righty:
      if S.is_empty():
        return False                          # nothing to match with
      if righty.index(c) != lefty.index(S.pop()):
        return False                          # mismatched
  return S.is_empty()                         # were all symbols matched?

3、测试一个函数是否有匹配标签

from array_stack import ArrayStack

def is_matched_html(raw):
  """Return True if all HTML tags are properly match; False otherwise."""
  S = ArrayStack()
  j = raw.find('<')               # find first '<' character (if any)
  while j != -1:
    k = raw.find('>', j+1)        # find next '>' character
    if k == -1:
      return False                # invalid tag
    tag = raw[j+1:k]              # strip away < >
    if not tag.startswith('/'):   # this is opening tag
      S.push(tag)                 
    else:                         # this is closing tag
      if S.is_empty():
        return False              # nothing to match with
      if tag[1:] != S.pop():   
        return False              # mismatched delimiter
    j = raw.find('<', k+1)        # find next '<' character (if any)
  return S.is_empty()             # were all opening tags matched?