用python设计一个使用二叉树实现数学表达式求值的程序，需要完成以下功能： （1）输入一个中缀表达式，建立二叉解析树，并对其进行先序、中序和后序的遍历；

好的，我可以为您提供一个 Python 实现的基于二叉树的数学表达式求值程序。

代码如下：

class Node:
    def __init__(self, value=None):
        self.left = None
        self.right = None
        self.value = value

    def __str__(self):
        return str(self.value)

def build_tree(expression):
    operator_stack = []
    operand_stack = []
    operators = set(['+', '-', '*', '/', '(', ')'])
    precedence = {'+': 1, '-': 1, '*': 2, '/': 2}
    for i in expression:
        if i == '(':
            operator_stack.append(i)
        elif i == ')':
            while operator_stack[-1] != '(':
                op = operator_stack.pop()
                r = operand_stack.pop()
                l = operand_stack.pop()
                node = Node(op)
                node.left = l
                node.right = r
                operand_stack.append(node)
            operator_stack.pop()
        elif i not in operators:
            operand_stack.append(Node(i))
        else:
            while operator_stack and operator_stack[-1] != '(' and precedence[i] <= precedence[operator_stack[-1]]:
                op = operator_stack.pop()
                r = operand_stack.pop()
                l = operand_stack.pop()
                node = Node(op)
                node.left = l
                node.right = r
                operand_stack.append(node)
            operator_stack.append(i)
    while operator_stack:
        op = operator_stack.pop()
        r = operand_stack.pop()
        l = operand_stack.pop()
        node = Node(op)
        node.left = l
        node.right = r
        operand_stack.append(node)
    return operand_stack.pop()

def preorder(node):
    if node:
        print(node, end=' ')
        preorder(node.left)
        preorder(node.right)

def inorder(node):
    if node:
        inorder(node.left)
        print(node, end=' ')
        inorder(node.right)

def postorder(node):
    if node:
        postorder(node.left)
        postorder(node.right)
        print(node, end=' ')

def evaluate(node):
    if node.left is None and node.right is None:
        return int(node.value)
    left_val = evaluate(node.left)
    right_val = evaluate(node.right)
    operator = node.value
    if operator == '+':
        return left_val + right_val
    elif operator == '-':
        return left_val - right_val
    elif operator == '*':
        return left_val * right_val
    elif operator == '/':
        return left_val / right_val

expression = input('请输入中缀表达式：')
tree = build_tree(expression)
print('先序遍历结果：', end='')
preorder(tree)
print()
print('中序遍历结果：', end='')
inorder(tree)
print()
print('后序遍历结果：', end='')
postorder(tree)
print()
print('表达式的值为：', evaluate(tree))

程序运行时，您需要输入一个中缀表达式，例如：`3 + 4 * (2 - 1)`，然后程序会根据中缀表达式构建二叉树，并输出先序、中序和后序遍历结果以及表达式的值。

希望这个程序能够帮助到您！