def preorder(self, treenode):
'前序（pre-order，NLR）遍历'
stack = []
while treenode or stack:
if treenode is not 0:
print treenode.data
stack.append(treenode)
treenode = treenode.left
else:
treenode = stack.pop()
treenode = treenode.right
def inorder(self, treenode):
'中序（in-order，LNR) 遍历'
stack = []
while treenode or stack:
if treenode:
stack.append(treenode)
treenode = treenode.left
else:
treenode = stack.pop()
print treenode.data
treenode = treenode.right
# def postorder(self, treenode):
# stack = []
# pre = 0
# while treenode or stack:
# if treenode:
# stack.append(treenode)
# treenode = treenode.left
# elif stack[-1].right != pre:
# treenode = stack[-1].right
# pre = 0
