Unique Binary Search Trees II

/**
* 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 n) {
vector<TreeNode *> ret;
if(n<1)
{
ret.push_back(NULL);
return ret;
}
return generate(1,n);
}
vector<TreeNode *> generate(int left,int right)
{
//generate() 产生left到right区间的bst
//返回bst的根节点vector
vector<TreeNode *> ret;
if(left>right)
{
ret.push_back(NULL);
return ret;
}
for(int i=left;i<=right;i++)
{
vector<TreeNode *> leftSubTree(generate(left,i-1));
vector<TreeNode *> rightSubTree(generate(i+1,right));
for(size_t j=0;j!=leftSubTree.size();j++)
{
for(size_t k=0;k!=rightSubTree.size();k++)
{
TreeNode * unit=new TreeNode(i);
unit->left=leftSubTree[j];
unit->right=rightSubTree[k];
ret.push_back(unit);
}
}
}
return ret;
}
};