145. Binary Tree Postorder Traversal

這種題一看就是不會(huì)做，哈哈哈，不會(huì)做還挺有理的。多寫幾遍，手熟就好。這種反轉(zhuǎn)preorder的想法還是挺奇妙的。

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