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

confused what  "{1,#,2,3}"  means? > read more on how binary tree is serialized on OJ.
再帰的にBSTを低から上へ順次確立する.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void build(vector<TreeNode*> &result, int nStart, int nEnd)
    {
        if (nStart > nEnd)
            result.push_back(NULL);
        
        else
        {
            for (int i = nStart; i <= nEnd; ++i)
            {
                vector<TreeNode*> left;
                build(left, nStart, i-1);
                vector<TreeNode*> right;
                build(right, i+1, nEnd);
                for (int j = 0; j < left.size(); ++j)
                {
                    for (int k = 0; k < right.size(); ++k)
                    {
                        TreeNode* root = new TreeNode(i);
                        root->left = left[j];
                        root->right = right[k];
                        result.push_back(root);
                    }
                }
            }
        }
    }
    
    vector<TreeNode *> generateTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<TreeNode*> result;
        build(result, 1, n);
        return result;
    }
};