二叉树的后序遍历

def solve():
    class TreeNode:
        def __init__(____):
            self.val = val
            self.left = left
            self.right = right
    
    root = TreeNode(____)
    root.left = TreeNode(____)
    root.right = TreeNode(____)
    root.left.left = TreeNode(____)
    root.left.right = TreeNode(____)
    
    result = []
    def postorder(____):
        if node:
            postorder(____)
            postorder(____)
            result.append(str(____))
    
    postorder(____)
    print('____'.join(____))