Inorder Traversal

中序遍历有两种，一种用栈辅助，一种是Morris Traversal

• Stack algorithm

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack, res = [], []
        while root or stack:
            if root:
                stack.append(root)
                root = root.left
            else:
                root = stack.pop()
                res.append(root.val)
                root = root.right
        return res

• Morris Traversal

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        pre, cur = root, None
        res = []

        while pre:
            cur = pre.left
            if cur:
                while cur.right and cur.right != pre:
                    cur = cur.right
                if not cur.right:
                    cur.right = pre
                    pre = pre.left
                    continue
                else:
                    cur.right = None
            res.append(pre.val)
            pre = pre.right

        return res