二叉树的前序遍历

def solve():
    class TreeNode:
        def __init__(____):
            self.val = val
            self.left = left
            self.right = right
    
    # 构建二叉树
    #       ____
    #      / \
    #     ____   ____
    #    / \
    #   ____   ____
    root = TreeNode(____)
    root.left = TreeNode(____)
    root.right = TreeNode(____)
    root.left.left = TreeNode(____)
    root.left.right = TreeNode(____)
    
    result = []
    def preorder(____):
        if node:
            result.append(str(____))
            preorder(____)
            preorder(____)
    
    preorder(____)
    print('____'.join(____))