20天【代码随想录算法训练营34期】第六章 二叉树 part06（● 654.最大二叉树 ● 617.合并二叉树 ● 700.二叉搜索树中的搜索 ● 98.验证二叉搜索树 ）

654.最大二叉树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def constructMaximumBinaryTree(self, nums: List[int]) -> Optional[TreeNode]:
        if not nums:
            return None

        max_num = max(nums)
        root = TreeNode(max_num)
        max_index = nums.index(max_num)
        left_nums = nums[:max_index]
        right_nums = nums[max_index+1:]
        root.left = self.constructMaximumBinaryTree(left_nums)
        root.right = self.constructMaximumBinaryTree(right_nums)

        return root

617.合并二叉树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
        if root1 is None:
            return root2
        if root2 is None:
            return root1
        root1.val += root2.val
        
        root1.left = self.mergeTrees(root1.left, root2.left)
        root1.right = self.mergeTrees(root1.right, root2.right)

        return root1

700.二叉搜索树中的搜索

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def searchBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
        if root is None:
            return None
        if root.val == val:
            return root
        
        if val < root.val:
            return self.searchBST(root.left, val)
        else:
            return self.searchBST(root.right, val)

98.验证二叉搜索树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def __init__(self):
        self.pre = None

    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        if root is None:
            return True
        left = self.isValidBST(root.left)

        if self.pre and self.pre.val >= root.val:
            return False
        else:
            self.pre = root

        right = self.isValidBST(root.right)

        return left and right