算法描述：

Given an integer n, generate all structurally unique BST‘s (binary search trees) that store values 1 ... n.

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST‘s shown below:

   1         3     3      2      1
    \       /     /      / \           3     2     1      1   3      2
    /     /       \                    2     1         2                 3

解題思路：深度優先，遞歸求解。

    vector<TreeNode*> generateTrees(int n) {
        vector<TreeNode*> resuts;
        if(n==0) return resuts;
        return generate(1,n);
    }
    
    vector<TreeNode*> generate(int start, int end){
        vector<TreeNode*> results;
        if(start > end){
            results.push_back(nullptr);
            
 
return results;
        }
        
        for(int i = start; i <= end; i++){
            vector<TreeNode*> left = generate(start,i-1);
            vector<TreeNode*> right = generate(i+1,end);
            for(auto l:left){
                for(auto r:right){
                    TreeNode
 
* temp = new TreeNode(i);
                    temp->left = l;
                    temp->right = r;
                    results.push_back(temp);
                }
            }
        }
        return results;
    }

LeetCode-95-Unique Binary Search Trees II