1382. Balance a Binary Search Tree

Problem Link

Description

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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

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Python3

# 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:
        A = []
 
        def dfs(node):
            if not node: return
 
            dfs(node.left)
            A.append(node.val)
            dfs(node.right)
        
        dfs(root)
        
        def build(left, right):
            if left > right: return None
 
            mid = (left + right) // 2
 
            node = TreeNode(A[mid])
            node.left = build(left, mid - 1)
            node.right = build(mid + 1, right)
 
            return node
 
        return build(0, len(A) - 1)