中序遍历二叉树

给定一个二叉树，返回它的中序 遍历。

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

输出: [1,3,2]

1. 递归

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

class Solution:
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res = []
        def recurse(node):
            if node:
                recurse(node.left)
                res.append(node.val)
                recurse(node.right)
        recurse(root)
        return res

2. 非递归

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

class Solution:
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root is None:
            return []
        stack = []
        res = []
        node = root
        while node or len(stack) > 0:
            if node:
                stack.append(node)
                node = node.left
            else:
                node = stack.pop()
                res.append(node.val)
                node = node.right
        return res