<p>Given an integer <code>n</code>, return <em>a list of all possible <strong>full binary trees</strong> with</em> <code>n</code> <em>nodes</em>. Each node of each tree in the answer must have <code>Node.val == 0</code>.</p>

<p>Each element of the answer is the root node of one possible tree. You may return the final list of trees in <strong>any order</strong>.</p>

<p>A <strong>full binary tree</strong> is a binary tree where each node has exactly <code>0</code> or <code>2</code> children.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://s3-lc-upload.s3.amazonaws.com/uploads/2018/08/22/fivetrees.png" style="width: 700px; height: 400px;" />
<pre>
<strong>Input:</strong> n = 7
<strong>Output:</strong> [[0,0,0,null,null,0,0,null,null,0,0],[0,0,0,null,null,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,null,null,null,null,0,0],[0,0,0,0,0,null,null,0,0]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 3
<strong>Output:</strong> [[0,0,0]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 20</code></li>
</ul>

/**
 * 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) {}
 * };
 */
#include <bits/stdc++.h>
#define ss second
#define ff first
#define pii pair<int,int>
#define pb push_back
#define ll long long
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<ll>	
#define pll pair<ll,ll>
#define vpll vector<pll>
#define vvll vector<vll>
#define ld long double
#define um unordered_map
#define all(x) x.begin(),x.end()



class Solution {
public:
    // vector<TreeNode*> foo(TreeNode* root ,ll n){
         
    // }
    vector<TreeNode*> allPossibleFBT(int n) {
        if(n==1){
            TreeNode* root = new TreeNode(0);
            return {root};
        }
        TreeNode* root = new TreeNode(0);
        vector<TreeNode*> res;

        for(ll l=1;l<=n-2;l+=2){
            vector<TreeNode*> kids_l = allPossibleFBT(l);
            vector<TreeNode*> kids_r = allPossibleFBT(n-1-l);

            for(auto a:kids_l){
                for(auto b:kids_r){
                    TreeNode* root1=new TreeNode(0);
                    root1->left=a;
                    root1->right=b;
                    res.pb(root1);
                }
            }
        }
        return res;
        
 
    }
};
42431