# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution94(object):              ######二叉樹中序遍歷的非遞迴方法，用棧
    def inorderTraversal(self, root):   #先把根節點入棧，如果左子樹一直不為空，就一直入棧，直到 
                                  ##把所有左節點入棧，然後pop棧頂元素，指標指向棧頂元素的右子樹
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack=[]
        res=[]
        if not root:
            return []
        while root or stack:
            while root:
                stack.append(root)
                root=root.left
            if stack:
                a=stack.pop()
                root=a.right
                res.append(a.val)
        return res