Python problem! builds the expression tree from expr. Do not change other functions. Test code is given at the end. Thank you very much!

class expressionTree: # constructTree(expr) builds the expression tree from expr # Here you can assume: # expr is an arithmetic expression with an answer value # The first non-space symbol is not "-" # (because we can add "0" to the left if so) # # getExpr() to reconstruct expr from the tree # # revPolish() to get the reverse polish notatation # # and maybe more...

class treeNode: def __init__(self, value, lchild, rchild): self.value, self.lchild, self.rchild = value, lchild, rchild

def __init__(self): self.treeRoot = None

#utility functions for constructTree def mask(self, s): # The function empties the inside of every outermost parentheses pair # and return it as a string # # e.g. s = ( 2 + 3^2 -(2-4)) + 2 - (-1 + 1/7) # It returns ( ) + 2 - ( ) # # --- code ----#

#Build expression tree from expr, set it to self.treeRoot def constructTree(self, expr): temp = 1 #input = the expression tree whose root is self.treeRoot #output = reconstructed arithmetic expression as a def getExpr(self): temp = 1 #input = the expression tree whose root is self.treeRoot #output = string that is the reverse polish notation def revPolish(self): temp = 1

# To check the mask function e = expressionTree() s =" ( 2 + 3*(2- 1/3))+ 2 - ((5-2/3)*3 - 2)" print(e.mask(s)) # should print # ( )+ 2 - ( )