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

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return []
        stack = []
        res = []
        p = root
        while p != None or len(stack) != 0:
            # First append all the left nodes,
            # if there's no left nodes to append,
            # consume p and append right nodes.
            if p != None:
                stack.append(p)
                p = p.left
            else:
                if len(stack)>0:
                    p = stack[-1]
                    del stack[-1]
                    res.append(p.val)
                    p = p.right
        return res

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

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return []
        stack = []
        res = []
        p = root
        while p != None or len(stack) != 0:
            # don't see curNode.left,
            # instead, see p.left
            while p != None:
                stack.append(p)
                p = p.left
            curNode = stack[-1]
            del stack[-1]
            res.append(curNode.val)
            p = curNode.right
        return res