Returns the number of Binary Search Trees for N.

GivenN, Generate all structurally uniqueBST's(Binary Search Trees) that store values 1...N.

For example,
GivenN= 3, your program shocould return all 5 unique BST's shown below.

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

 

Confused what"{1,#,2,3}"Means? > Read more on how binary tree is serialized on OJ.

 

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {private:    vector<TreeNode *> generate(int l, int r)    {        vector<TreeNode *> ans;         if (l > r)        {            ans.push_back(NULL);        }        else        {            for (int i=l; i<=r; i++){                vector<TreeNode *> left = generate(l, i-1);                vector<TreeNode *> right = generate(i+1, r);                for (int j=0; j<left.size(); j++)                {                   for (int k=0; k<right.size(); k++)                    {                        TreeNode *pRoot = new TreeNode(i);                        pRoot->left = left[j];                        pRoot->right = right[k];                        ans.push_back(pRoot);                    }                }            }                    }                return ans;    }public:    vector<TreeNode *> generateTrees(int n) {        return generate(1, n);    }};

 

Returns the number of Binary Search Trees for N.