145. Binary Tree Postorder Traversal
这种题一看就是不会做，哈哈哈，不会做还挺有理的。多写几遍，手熟就好。这种反转preorder的想法还是挺奇妙的。
class Solution(object):
def postorderTraversal(self, root):
"""
:type root: TreeNode
:rtype: List[int]
"""
result = []
stack = []
p = root
while stack or p:
if p:
stack.append(p)
result = [p.val] + result # Reverse the process of preorder
p = p.right # Reverse the process of preorder
else:
node = stack.pop()
p = node.left # Reverse the process of preorder
return result
