Leetcode108: 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 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*> createTree(int start, int end)
{
vector<TreeNode*> res;
if(start>end)
{
res.push_back(NULL);
return res;
}
for(int i = start; i <= end; i++)
{
vector<TreeNode*> left = createTree(start, i-1);
vector<TreeNode*> right = createTree(i+1, end);
for(int j = 0; j < left.size(); j++)
{
for(int k = 0; k < right.size(); k++)
{
TreeNode* root = new TreeNode(i);
root->left = left[j];
root->right = right[k];
res.push_back(root);
}
}
}
return res;
}
vector<TreeNode*> generateTrees(int n) {
return createTree(1, n);
}
};