二叉树前序遍历

def solve():
    class TreeNode:
        def __init__(self, val):
            self.val = val
            self.left = None
            self.right = None
    root = TreeNode(1)
    root.left = TreeNode(2)
    root.right = TreeNode(3)
    root.left.left = TreeNode(4)
    root.left.right = TreeNode(5)
    result = []
    def preorder(node):
        if node:
            result.append(str(node.val))
            preorder(node.left)
            preorder(node.right)
    preorder(root)
    print(' '.join(result))