95. Unique Binary Search Trees II

https://leetcode.com/problems/unique-binary-search-trees-ii/

Problem

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

Constraints:

• 0 <= n <= 8

Solution

/**
 * 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*> generateTrees(int n) {
        if (n == 0) return vector<TreeNode*>();
        return generateTrees(1, n);
    }
    
    vector<TreeNode*> generateTrees(int start, int end) {
        if (start == end) return vector<TreeNode*>{new TreeNode(start)};
        vector<TreeNode*> trees;
        for (int i = start; i <= end; ++i) {
            vector<TreeNode*> left = (i == start) ? vector<TreeNode*>{nullptr} : generateTrees(start, i - 1);
            vector<TreeNode*> right = (i == end) ? vector<TreeNode*>{nullptr} :  generateTrees(i + 1, end);
            for (const auto &l: left) {
                for (const auto &r: right) {
                    TreeNode *root = new TreeNode(i);
                    root->left = l;
                    root->right = r;
                    trees.emplace_back(root);
                }
            }
        }
        return trees;
    }
};

• #tree