二叉树的中序遍历(非递归)

思想：
1）借助栈
2）stack一直添加非空的叶子结点
3）stack每次pop（）结点p，然后访问，最后p.right更新root

# 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: TreeNode):
        stack = []
        res = []
        while root!=None or len(stack)>0:
            while root!=None:
                stack.append(root)
                root = root.left
            if len(stack)>0:
                p = stack.pop()
                res.append(p.val)
                root = p.right
        return res