LeetCode-M-Unique Binary Search Trees II

##題意
Given an integer 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

解法

實現

/**
 * 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*> generateTreesHelper(int start, int end){
        vector<TreeNode*> result;
        if(start > end) {
            result.push_back(NULL);
        }else if(start == end){
            TreeNode* root = new TreeNode(start);
            result.push_back(root);
        }else{
            for(int i = start; i <= end; ++i){
                vector<TreeNode*> left = generateTreesHelper(start,i-1);
                vector<TreeNode*> right = generateTreesHelper(i+1,end);
                for(int l = 0; l < left.size(); ++l){
                    for(int r = 0; r < right.size(); ++r){
                        TreeNode* root = new TreeNode(i);
                        root->left = left[l];
                        root->right = right[r];
                        result.push_back(root);
                    }
                }
            }
        }
        return result;
    }
    vector<TreeNode*> generateTrees(int n) {
        vector<TreeNode*> ivec;
        if(n <= 0) return ivec;
        ivec = generateTreesHelper(1,n);
        return ivec;
    }
};