Split BST — LeetCode 776 Python Solution

The problem

Given the root of a binary search tree (BST) and an integer target, split the tree into two subtrees where the first subtree has nodes that are all smaller or equal to the target value, while the second subtree has all nodes that are greater than the target value. It is not necessarily the case that the tree contains a node with the value target.

Example

Input

root = [4,2,6,1,3,5,7], target = 2

Output

[[2,1],[4,3,6,null,null,5,7]]

Python solution

# 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 splitBST(
        self, root: Optional[TreeNode], target: int
    ) -> List[Optional[TreeNode]]:
        def dfs(root):
            if root is None:
                return [None, None]
            if root.val <= target:
                l, r = dfs(root.right)
                root.right = l
                return [root, r]
            else:
                l, r = dfs(root.left)
                root.left = r
                return [l, root]

        return dfs(root)

Complexity

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 776. Split BST is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Tree, Binary Tree and Binary Search Tree.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

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