894

/**
 * 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 {
unordered_map<int, vector<TreeNode*>> memo;
public:
    vector<TreeNode*> allPossibleFBT(int n) {
        vector<TreeNode*> ans;
        if(memo.count(n)) return memo[n];
        if(n == 1){
            TreeNode *node = new TreeNode(0);
            ans.push_back(node);
            memo[n] = ans;
        }
        else{
            for(int i=1; i<=n-2; i+=2){
                vector<TreeNode*> left = allPossibleFBT(i);
                vector<TreeNode*> right = allPossibleFBT(n-i-1);
                for(int i=0; i<left.size(); i++){
                    for(int j=0; j<right.size(); j++){
                        TreeNode *node = new TreeNode(0);
                        node->left = left[i];
                        node->right = right[j];
                        ans.push_back(node);
                    }
                }
            }
            memo[n] = ans;
        }
        return memo[n];
    }
};