[LeetCode] Unique Binary Search Trees II （难以忍受的递归）

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

class Solution {
private:
    vector<TreeNode*> generateTreesRec(int start, int end){
        vector<TreeNode*> v;
        if(start > end){
            v.push_back(NULL);
            return v;
        }
        for(int i = start; i <= end; ++i){
            vector<TreeNode*> left = generateTreesRec(start, i - 1);
            vector<TreeNode*> right = generateTreesRec(i + 1, end);
            TreeNode *node;
            for(int j = 0; j < left.size(); ++j){
                for(int k = 0; k < right.size(); ++k){
                    node = new TreeNode(i);
                    node->left = left[j];
                    node->right = right[k];
                    v.push_back(node);
                }
            }
        }
        return v;
    }
public:
    vector<TreeNode *> generateTrees(int n) {
        return generateTreesRec(1, n);
    }
};