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.
[1, 10^4]1 <= Node.val <= 10^5root = [1,null,2,null,3,null,4,null,null][2,1,3,null,null,null,4]root = [2,1,3][2,1,3]