894. All Possible Full Binary Trees

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

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

A full binary tree is a binary tree where each node has exactly 0 or 2 children.

/**
 * 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 & 1)) return {};
        if(n == 1) return {new TreeNode()};
        vector<TreeNode*> res;
        vector<vector<TreeNode*>> childs;
        for(int l = 1; l < n - 1; l += 2) {
            vector<TreeNode*> child = allPossibleFBT(l);
            childs.push_back(child);
        }
        
        for(int i = 0; i < childs.size(); i++) {
            for(auto& ll : childs[i]) {
                for(auto& rr : childs[childs.size() - 1 - i]) {
                    res.push_back(new TreeNode(0, ll, rr));
                }
            }
        }
        
        return res;
    }
};