#### 方法一：递归

``````/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode() : val(0), left(nullptr), right(nullptr) {}
*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<TreeNode*> allPossibleFBT(int N) {
if(N % 2 == 0) return {};
if(N == 1) return {new TreeNode(0)};

vector<TreeNode*> ans;
for(int i = 1;i < N;i += 2)
{
for(auto l:allPossibleFBT(i))
{
for(auto r:allPossibleFBT(N-i-1))
{
TreeNode* root = new TreeNode(0);
root->left = l;
root->right = r;

ans.push_back(root);
}
}
}

return ans;
}
};``````

#### 方法二 递归加记忆化

``````/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode() : val(0), left(nullptr), right(nullptr) {}
*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<TreeNode*> allPossibleFBT(int N) {
if(N % 2 == 0) return {};
if(N == 1) return {new TreeNode(0)};

vector<TreeNode*> ans;
for(int i = 1;i < N;i += 2)
{
for(auto l:allPossibleFBT(i))
{
for(auto r:allPossibleFBT(N-i-1))
{
TreeNode* root = new TreeNode(0);
root->left = l;
root->right = r;

ans.push_back(root);
}
}
}

return ans;
}
};``````