如何从字符串表达式构建二叉树

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def buildTree(expression):
    if not expression:
        return None
    
    operators = ['+', '-', '*', '/']
    for i in range(len(expression)):
        if expression[i] in operators:
            root = TreeNode(expression[i])
            root.left = buildTree(expression[:i])
            root.right = buildTree(expression[i+1:])
            return root
    
    return TreeNode(expression)

# 示例用法
expression = "3*4+5"
root = buildTree(expression)