All Possible Full Binary Trees

给一个数字n, 求n个nodes组成的二叉树.

这里注意几个corn cases, 比如0,1,2

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<TreeNode>[] lists = new List[21];
    public List<TreeNode> allPossibleFBT(int n) {
        List<TreeNode> res = new LinkedList();
        if(n <= 0)
            return res;
        if(n == 1){
            res.add(new TreeNode(0));
            return res;
        }
        if(n % 2 != 1){
            return res;
        }
        if(lists[n] != null){
            return lists[n];
        }
        for(int i = 0; i < n; i++) {
            int j = n - i - 1;
            for (TreeNode left: allPossibleFBT(i))
                for (TreeNode right: allPossibleFBT(j)) {
                    TreeNode node = new TreeNode(0);
                    node.left = left;
                    node.right = right;
                    res.add(node);
                }
            }
        lists[n] = res;
        return res;
    }
}