All_Possible_Full_Binary_Trees.cpp

/**
 * 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)};
        }
      if (const auto it = memo.find(n); it != memo.cend()) {
        return it->second;
      }

      vector<TreeNode*> anwser;

      for (int leftCount = 0; leftCount < n; ++leftCount) {
        const int rightCount = n - 1 - leftCount;
      
        for (TreeNode* left : allPossibleFBT(leftCount)) {
          for (TreeNode* right : allPossibleFBT(rightCount)){
            anwser.push_back(new TreeNode(0));
            anwser.back()->left = left;
            anwser.back()->right = right;
          }
        }
      }

      return memo[n] = anwser;
        
    }

private:
    unordered_map<int, vector<TreeNode*>> memo;
};