1 class Solution {     2      3     private Map
     
      > memo; 4      5     public List
      
        allPossibleFBT(int N) { 6         this.memo = new HashMap<>(); 7         return backtrack(N); 8     } 9     10     private List
       
         backtrack(int n) {11         List
        
          ret = new ArrayList<>();12         if (n < 1) {13             return ret;14         }15         16         if (n == 1) {17             ret.add(new TreeNode(0));18             return ret;19         }20         21         if (memo.containsKey(n)) {22             return memo.get(n);23         }24         25         for (int i = 1; i < n; i++) {26             for (TreeNode left : backtrack(i)) {27                 for (TreeNode right : backtrack(n - i - 1)) {28                     TreeNode node = new TreeNode(0);29                     node.left = left;30                     node.right = right;31                     ret.add(node);32                 }33             }34         }35         36         memo.put(n, ret);37         return ret;38     }39 }