二叉树的前序遍历

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

def solve():
    def preorderTraversal(root):
        result = []
        def dfs(node):
            if node:
                result.append(node.val)
                dfs(node.left)
                dfs(node.right)
        dfs(root)
        return result
    root = TreeNode(1, None, TreeNode(2, TreeNode(3)))
    print(preorderTraversal(root))