144. Binary Tree Preorder Traversal刷题笔记

问题描述 前序遍历。注意嵌套函数里的list应该用append而不是+=来添加元素

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        result = []
        def Preoder(root):
            if root:
                result.append(root.val)
                Preoder(root.left)
                Preoder(root.right)
        Preoder(root)
        return result

运行结果：