def postorder_nonrec(t, proc):
s = SStack()
while t is not None or not s.is_empty():
while t is not None: # iterate until top has no child
s.push(t)
t = t.left if t.left is not None else t.right
# if we can go left, go, otherwise, go right
t = s.pop() # get the node to be access
proc(t.data)
if not s.is_empty() and s.top().left == t:
t = s.top().right # end of left visit, turn right
else:
t = None # end of right visit, force to backtrack
def trans_infix_suffix(line):
st = SStack()
llen = len(line)
exp = []
for x in tokens(line): # tokens 是一个待定义的生成器
if x not in infix_operators: # 运算对象直接送出
exp.append(x)
elif st.is_empty() or x == '(': # 左括号进栈
st.push(x)
elif x == ')': # 处理右括号的分支
while not st.is_empty() and st.top() != "(":
exp.append(st.pop())
if st.is_empty(): # 没找到左括号，就是不配对
raise SyntaxError("Missing \'(\'.")
st.pop() # 弹出左括号，右括号也不进栈
else: # 处理算术运算符，运算符都看作是左结合
while (not st.is_empty() and priority[st.top()] >= priority[x]):
exp.append(st.pop())
st.push(x) # 算术运算符进栈
while not st.is_empty(): # 送出栈里剩下的运算符
if st.top() == "(": # 如果还有左括号，就是不配对
raise SyntaxError("Extra \'(\' in expression.")
exp.append(st.pop())
return exp