BM23 二叉树的前序遍历

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param root TreeNode类 
# @return int整型一维数组
#
class Solution:
    ans=list()
    def preorderTraversal(self , root: TreeNode) -> List[int]:
        # write code here
            if root is None:
                return None
            
            self.ans.append(root.val)
            if root.left is not None:
                self.preorderTraversal(root.left)
            if root.right is not None:
                self.preorderTraversal(root.right)
            return self.ans