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/**
 * Definition for a binary tree node.
 * 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) {
        vector<TreeNode*> result;
        if (n <= 0) return result;
        else return getSubTree(1, n);
    }
    vector<TreeNode*> getSubTree(int s, int e) {
        vector<TreeNode*> ret;
        if (e < s) {
            ret.push_back(NULL);
        }
        if (e == s) {
            ret.push_back(new TreeNode(s));
        }
        if (e > s) {
            for(int i = s; i <= e; i++) {
                vector<TreeNode*> l = getSubTree(s, i-1);
                vector<TreeNode*> r = getSubTree(i+1, e);
                for(int j = 0; j < l.size(); j++) {
                    for(int j2 = 0; j2 < r.size(); j2++) {
                        TreeNode* nd = new TreeNode(i);
                        nd->left = l[j];
                        nd->right = r[j2];
                        ret.push_back(nd);
                    }
                }
            }
        }
        return ret;
    }
};