[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

哎，WA了两次，因为把root = new TreeNode(i)放错了位置，这句应该放在最里面的循环里。因为每次都要新生成一棵树！

/**
 * 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 st, int ed) {
        vector<TreeNode *> res;
        TreeNode *root = NULL;
        if (st > ed) {
            res.push_back(root);
            return res;
        }
        vector<TreeNode *> left;
        vector<TreeNode *> right;
        for (int i = st; i <= ed; ++i) {
            left = generateTrees(st, i - 1);
            right = generateTrees(i + 1, ed);
            for (int j = 0; j < left.size(); ++j) {
                for (int k = 0; k < right.size(); ++k) {
                    root = new TreeNode(i);
                    root->left = left[j];
                    root->right = right[k];
                    res.push_back(root);
                }
            }
        }
        return res;
    }
    
    vector<TreeNode *> generateTrees(int n) {
        return generateTrees(1, n);
    }
};