修剪二叉树

# 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 trimBST(self, root: TreeNode, low: int, high: int) -> TreeNode:
        '''
        确认递归函数参数以及返回值：返回更新后剪枝后的当前root节点
        '''
        # Base Case
        if not root: return None

        # 单层递归逻辑
        if root.val < low:
            # 若当前root节点小于左界：只考虑其右子树，用于替代更新后的其本身，抛弃其左子树整体
            return self.trimBST(root.right, low, high)
        
        if high < root.val:
            # 若当前root节点大于右界：只考虑其左子树，用于替代更新后的其本身，抛弃其右子树整体
            return self.trimBST(root.left, low, high)

        if low <= root.val <= high:
            root.left = self.trimBST(root.left, low, high)
            root.right = self.trimBST(root.right, low, high)
            # 返回更新后的剪枝过的当前节点root
            return root