leetcode 94
使用栈，非递归解法：
class Solution(object):
def inorderTraversal(self, root):
"""
:type root: TreeNode
:rtype: List[int]
"""
if not root:
return []
stack, cur = [root], root
result = []
while stack:
if cur:
stack.append(cur)
cur = cur.left
else:
cur = stack.pop()
result.append(cur.val)
cur = cur.right
return result[:-1]
leetcode 144
使用栈，非递归解法：
class Solution(object):
def preorderTraversal(self, root):
"""
:type root: TreeNode
:rtype: List[int]
"""
if not root:
return []
stack, result = [root], []
while stack:
cur = stack.pop()
result.append(cur.val)
if cur.right:
stack.append(cur.right)
if cur.left:
stack.append(cur.left)
return result
