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 generateTrees(n: number): Array<TreeNode | null> {
    if (n == 0) return [];
    return helper(1, n);
}

function helper(start: number, end: number): Array<TreeNode | null> {
    let ans = [];
    if (start > end) {
        ans.push(null);
        return ans;
    }
    for (let i = start; i <= end; i++) {
        let lefts = helper(start, i - 1);
        let rights = helper(i + 1, end);
        for (let left of lefts) {
            for (let right of rights) {
                let root = new TreeNode(i, left, right);
                ans.push(root);
            }
        }
    }
    return ans;
}