[leetcode] 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 {
private:
vector<TreeNode*> generate(int start,int end){
vector<TreeNode*> subTree;
if(start>end){
subTree.push_back(nullptr);
return subTree;
}
for(int k=start;k<=end;k++){
vector<TreeNode*> leftSubs=generate(start,k-1);
vector<TreeNode*> rightSubs=generate(k+1,end);
for(auto i:leftSubs){
for(auto j:rightSubs){
TreeNode *node=new TreeNode(k);
node->left=i;
node->right=j;
subTree.push_back(node);
}
}
}
return subTree;
}
public:
vector<TreeNode *> generateTrees(int n) {
if(n==0){
return generate(1,0);
}
return generate(1,n);
}
};