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1382. Balance a Binary Search Tree

Difficulty Topics

Description

Given the root of a binary search tree, return a balanced binary search tree with the same node values. If there is more than one answer, return any of them.

A binary search tree is balanced if the depth of the two subtrees of every node never differs by more than 1.

 

Example 1:

Input: root = [1,null,2,null,3,null,4,null,null]
Output: [2,1,3,null,null,null,4]
Explanation: This is not the only correct answer, [3,1,4,null,2] is also correct.

Example 2:

Input: root = [2,1,3]
Output: [2,1,3]

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • 1 <= Node.val <= 105

Solution

balance-a-binary-search-tree.py
# 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 balanceBST(self, root: TreeNode) -> TreeNode:

        def storeNodes(root, arr):
            if not root: return

            storeNodes(root.left,arr)
            arr.append(root.val)
            storeNodes(root.right,arr)

        def build(arr):
            if not arr: return None

            mid = len(arr) // 2
            root = TreeNode(arr[mid])
            root.left = build(arr[:mid])
            root.right = build(arr[mid+1:])

            return root

        arr = []
        storeNodes(root,arr)

        return build(arr)