Unique Binary Search Trees II

给一个数, 返回所有可能的BST. 找到所有的卡塔兰数组合.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
        return generate(1,n);
    }
    public ArrayList<TreeNode> generate (int start, int end) {
        ArrayList<TreeNode> res = new ArrayList<TreeNode>();
        if(start > end){
            res.add(null);
            return res;
        }
        for(int i = start; i <= end; i++) {
            ArrayList<TreeNode> left = generate(start, i - 1);
            ArrayList<TreeNode> right = generate(i+1, end);
            for(TreeNode l : left){
                for(TreeNode r : right) {
                    TreeNode root = new TreeNode(i);
                    root.left = l;
                    root.right = r;
                    res.add(root);
                }
            }
        }
        return res;
    }
}