144，145 二叉树的前序，后序遍历（中等，困难）

 给定一个二叉树，返回它的 前序 遍历。

 示例:

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

输出: [1,2,3]

# 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]
        """
        def dummy(root,l):
            if not root:
                return 
            result.append(root.val)
            if root.left:
                dummy(root.left,result)
            if root.right:
                dummy(root.right,result)
        result=[]
        dummy(root,result)
        return result

执行用时: 44 ms, 在Binary Tree Preorder Traversal的Python3提交中击败了98.03% 的用户

 

给定一个二叉树，返回它的 后序 遍历。

示例:

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

输出: [3,2,1]

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

class Solution:
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        def dummy(root,l):
            if not root:
                return
            if root.left:
                dummy(root.left,result)
            if root.right:
                dummy(root.right,result)
            result.append(root.val)
        result=[]
        dummy(root,result)
        return result

执行用时: 44 ms, 在Binary Tree Postorder Traversal的Python3提交中击败了97.88% 的用户