leetcode 173 二叉搜索树迭代器

思路：二叉树的中序遍历是有序的！！！那就好搞了啊，回顾二叉树的中序遍历，栈的写法，照搬就好了。

先把所有左节点丢到栈中，获取最小值时弹栈，然后将指针指向其右节点，入栈，如果右节点有左子节点，再全部入栈。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class BSTIterator(object):

    def __init__(self, root):
        """
        :type root: TreeNode
        """
        self.stack = []
        while root:
            self.stack.append(root)
            root = root.left

    def next(self):
        """
        @return the next smallest number
        :rtype: int
        """
        top = self.stack.pop()
        node = top.right
        while node:
            self.stack.append(node)
            node = node.left
        return top.val


    def hasNext(self):
        """
        @return whether we have a next smallest number
        :rtype: bool
        """
        return self.stack != []


# Your BSTIterator object will be instantiated and called as such:
# obj = BSTIterator(root)
# param_1 = obj.next()
# param_2 = obj.hasNext()

嗯，栈和二叉树先这样，明天想写点 dp 和贪心的了。