class Solution:
    # @param root, a tree node
    # @return a list of integers
    def inorderTraversal(self, root):
        res = []
        node = root
        stack = []
        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