leetcode894

 class Solution {    

     private Map<Integer, List<TreeNode>> memo;

     public List<TreeNode> allPossibleFBT(int N) {
         this.memo = new HashMap<>();
         return backtrack(N);
     }

     private List<TreeNode> backtrack(int n) {
         List<TreeNode> ret = new ArrayList<>();
         if (n < 1) {
             return ret;
         }

         if (n == 1) {
             ret.add(new TreeNode(0));
             return ret;
         }

         if (memo.containsKey(n)) {
             return memo.get(n);
         }

         for (int i = 1; i < n; i++) {
             for (TreeNode left : backtrack(i)) {
                 for (TreeNode right : backtrack(n - i - 1)) {
                     TreeNode node = new TreeNode(0);
                     node.left = left;
                     node.right = right;
                     ret.add(node);
                 }
             }
         }

         memo.put(n, ret);
         return ret;
     }
 }