leetcode[95]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

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