Leetcode: Unique Binary Search Trees II

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

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

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

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<TreeNode *> generateTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return generateTrees(1, n);
    }
    
     vector<TreeNode *> generateTrees(int start, int end) 
    {
        // If the range is invalid, then return an empty tree
        vector<TreeNode *> result;
        
        if (start > end) 
        {
            result.push_back(NULL);
            return result;
        }

        
        for (int i = start; i <= end; ++i) {
            // Generate all possible left subtrees (including the empty subtree)
            vector<TreeNode *> leftSubtrees = generateTrees(start, i - 1);

            // Generate all possible right subtrees (including the empty subtree)
            vector<TreeNode *> rightSubtrees = generateTrees(i + 1, end);

            // For each left subtree and each right subtree, create a root node
            // with value i and then link the two subtrees to the root
            for (int j = 0; j < leftSubtrees.size(); ++j) {
                for (int k = 0; k < rightSubtrees.size(); ++k) {
                    TreeNode *root = new TreeNode(i);
                    root->left = leftSubtrees[j];
                    root->right = rightSubtrees[k];
                    result.push_back(root);
                }
            }
        }
        return result;
    }
    
};