LeetCode 1382. Balance a Binary Search Tree

Question

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.

https://leetcode.com/problems/balance-a-binary-search-tree/

Solution

• Solution1

class Solution {
    public TreeNode balanceBST(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        dfs(root, res);
        return bulid(0, res.size() - 1, res);
    }
    private void dfs(TreeNode root, List<Integer> res){
        if (root == null) return;
        dfs(root.left, res);
        res.add(root.val);
        dfs(root.right, res);
    }
    private TreeNode bulid(int left, int right, List<Integer> res){
        if (left > right) return null;
        int mid = left + (right - left)/ 2;
        TreeNode root = new TreeNode(res.get(mid));
        root.left = bulid(left, mid - 1, res);
        root.right = bulid(mid + 1, right, res);
        return root;
    }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)