def infix_to_postfix(infixexpr):
#首先用一個dict來設定各個運算符的優先級
prec = {}
prec["*"] = 2 #AND
prec["+"] = 2 #OR
prec["-"] = 2 #NAND
prec["("] = 1
#step one 創建一個新的stack和一個空的list
opStack = Stack()
postfixList = []
#將string轉成list
tokenList = list(infixexpr)
for token in tokenList:
if token in "ABCDEFGHIJKLMNOPQRSTUVWXYZ" or token in "0123456789": #檢查是運算元還是運算符
postfixList.append(token)
elif token == '(':
opStack.push(token)
elif token == ')':
topToken = opStack.pop()
while topToken != '(':
postfixList.append(topToken)
topToken = opStack.pop()
else:
while (not opStack.is_empty()) and \
(prec[opStack.Top()] >= prec[token]):
postfixList.append(opStack.pop())
opStack.push(token)
#最後將stack清空
while not opStack.is_empty():
postfixList.append(opStack.pop())
return "".join(postfixList)
