02. Unique Binary Search Trees II.cpp

/*
Unique Binary Search Trees II
=============================

Given an integer n, return all the structurally unique BST's (binary search trees), which has exactly n nodes of unique values from 1 to n. Return the answer in any order.

Example 1:
Input: n = 3
Output: [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]

Example 2:
Input: n = 1
Output: [[1]]
 
Constraints:
1 <= n <= 8
*/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

class Solution
{
public:
    vector<TreeNode *> dfs(int start, int end)
    {
        if (start > end)
            return {NULL};
        if (start == end)
            return {new TreeNode(start)};

        vector<TreeNode *> ans;

        for (int i = start; i <= end; ++i)
        {
            auto left = dfs(start, i - 1);
            auto right = dfs(i + 1, end);

            for (auto &l : left)
            {
                for (auto &r : right)
                {
                    TreeNode *root = new TreeNode(i);
                    root->left = l;
                    root->right = r;
                    ans.push_back(root);
                }
            }
        }

        return ans;
    }

    vector<TreeNode *> generateTrees(int n)
    {
        return dfs(1, n);
    }
};