98. Validate Binary Search Tree

https://leetcode.com/problems/validate-binary-search-tree/

solution

• 注意二叉搜索树定义：根节点大于所有左子树，小于所有右子树。单个树递归的做法是无法满足定义的

• 递归也可以采用直观解法思路：中序展开为数组，判断数组是否排好序

class Solution:
    def __init__(self):
        self.pre = None

    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        if not root:
            return True

        # 注意左右需要返回
        left = self.isValidBST(root.left)

        if self.pre is not None and self.pre >= root.val:
            return False
        self.pre = root.val

        right = self.isValidBST(root.right)
        return left and right

时间复杂度：O(n) 空间复杂度：O(h)

• 层序遍历

• 后序遍历

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        def bst(root, min_val=float('-inf'), max_val=float('inf')):
            if root == None:
                return True

            if not (min_val < root.val < max_val):
                return False

            return (bst(root.left, min_val, root.val) and
                    bst(root.right, root.val, max_val))

        return bst(root)

follow up

958. Check Completeness of a Binary Tree

• bfs: 一个是否最后一层的flag (节点个数), 一个是否应该stop的flag (是否有空的左或右节点)

# https://zhuanlan.zhihu.com/p/360523724

class Solution:
    def isCompleteTree(self, root: Optional[TreeNode]) -> bool:
        queue = collections.deque()
        queue.append(root)

        while queue:
            node = queue.popleft()

            if node is None:
                break  # 需要完全退出循环，如果通常bfs按层，还有一层for导致有问题

            queue.append(node.left)
            queue.append(node.right)

        for i in queue:
            if i is not None:
                return False
        return True

• bfs: None也加入到queue中, 最后把None pop出去，检查是否还有额外node.

• bfs: 记录层序的上一个node, 如果现在node非空，上一个node为None,则False

class Solution:
    def isCompleteTree(self, root: Optional[TreeNode]) -> bool:
        if not root:
            return True

        queue = collections.deque([root])
        while queue[0] is not None:
            node = queue.popleft()
            # 显著不同是: 把None也加进去, 注意判断条件的改变
            queue.append(node.left)
            queue.append(node.right)

        for node in queue:
            if node:
                return False
        return True

class Solution:
    def isCompleteTree(self, root: Optional[TreeNode]) -> bool:
        if not root:
            return True

        queue = collections.deque()
        queue.append(root)

        while queue:
            for _ in range(len(queue)):
                node = queue.popleft()

                if node is None:
                    if any([item is not None for item in queue]):
                        return False
                else:
                    queue.append(node.left)
                    queue.append(node.right)
        return True