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;
    }
}