Unique Binary Search Trees II

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------QUESTION------

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

------SOLUTION------

class Solution {public:    vector<TreeNode*> generateTrees(int n) {        vector<TreeNode*> result;        if(n==0)        {            result.push_back(NULL);            return result;        }        recursion(1,n, result);        return result;    }        void recursion(int start, int end, vector<TreeNode*> &root)    {        if(start == end)         {            root.push_back(new TreeNode(start));            return;        }                for(int i = start; i<=end; i++)         {               vector<TreeNode*> leftTree;            vector<TreeNode*> rightTree;            if(i > start)             {                recursion(start, i-1, leftTree);            }            if(i < end)            {                recursion(i+1, end, rightTree);            }            if(leftTree.empty())            {                for(int j = 0; j< rightTree.size(); j++)                {                    TreeNode* newRoot = new TreeNode(i);                     newRoot->right = rightTree[j];                    root.push_back(newRoot);                 }            }            else if(rightTree.empty())            {                 for(int j = 0; j< leftTree.size(); j++)                 {                     TreeNode* newRoot = new TreeNode(i);                      newRoot->left = leftTree[j];                     root.push_back(newRoot);                 }            }            else{                 for(int j = 0; j< leftTree.size(); j++)                 {                     for(int k = 0; k< rightTree.size(); k++)                     {                         TreeNode* newRoot = new TreeNode(i);                         newRoot->left = leftTree[j];                         newRoot->right = rightTree[k];                         root.push_back(newRoot);                     }                 }            }        }      }};

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