Q:

給定一個二叉樹，返回它的 前序 遍歷。

 示例:

輸入: [1,null,2,3]  
   1
    \
     2
    /
   3 

輸出: [1,2,3]

連結：https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/

思路：遞迴 前序遍歷二叉樹

程式碼：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return []
        else:
            res = []
            self.pre_digui(root, res)
        return res
    
    def pre_digui(self,root,res):
        if root == None:
            return
        res.append(root.val)
        self.pre_digui(root.left,res)
        self.pre_digui(root.right,res)