LeetCode - Unique Binary Search Trees II

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

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:

vector<TreeNode *> generateTrees(int begin,int end){
vector<TreeNode *> rootVec,leftVec,rightVec;
if(begin>end){
rootVec.push_back(NULL);
return rootVec;
}

for(int k=begin;k<=end;k++){
leftVec=generateTrees(begin,k-1);
rightVec=generateTrees(k+1,end);
for(int i=0;i<leftVec.size();i++){
for(int j=0;j<rightVec.size();j++){
TreeNode *root=new TreeNode(k);
root->left=leftVec[i];
root->right=rightVec[j];
rootVec.push_back(root);
}
}
}
return rootVec;
}

vector<TreeNode *> generateTrees(int n) {
return generateTrees(1,n);
}
};