leetcode 95. 不同的二叉搜索树 II

[code]class Solution {
public:
vector<TreeNode*> generateTrees(int n) {
if (not n) return vector<TreeNode*>{};
return _dfs(1, n);

}
vector<TreeNode*> _dfs(int start, int end){
vector<TreeNode*> res;
if(start > end){
res.push_back(NULL);
return res;
}else{
for(int i = start; i <= end; i++){
vector<TreeNode*> left = _dfs(start, i - 1);
vector<TreeNode*> right = _dfs(i + 1, end);
for(TreeNode* l: left)
for(TreeNode* r: right){
TreeNode* t = new TreeNode(i);
t->left = l;
t->right = r;
res.push_back(t);
}
}
}
return res;
}
};