Solution.ts

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function balanceBST(root: TreeNode | null): TreeNode | null {
    const nums: number[] = [];
    const dfs = (root: TreeNode | null): void => {
        if (root == null) {
            return;
        }
        dfs(root.left);
        nums.push(root.val);
        dfs(root.right);
    };
    const build = (i: number, j: number): TreeNode | null => {
        if (i > j) {
            return null;
        }
        const mid: number = (i + j) >> 1;
        const left: TreeNode | null = build(i, mid - 1);
        const right: TreeNode | null = build(mid + 1, j);
        return new TreeNode(nums[mid], left, right);
    };
    dfs(root);
    return build(0, nums.length - 1);
}