94. 二叉树的中序遍历

给定一个二叉树的根节点 root ，返回它的 中序 遍历。
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

Python 解答：
1.递归

# 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 inorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        def inOrder(root):
            if not root:
                return 
            else:
                inOrder(root.left)
                res.append(root.val)
                inOrder(root.right)
        inOrder(root)
        return res

2.非递归

# 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 inorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        p = root
        stack = []
        while p or stack:
            if p:
                stack.append(p)
                p = p.left
            else:
                temp = stack.pop()
                res.append(temp.val)
                if temp.right:
                    p = temp.right
        return res