Java实现-不同的二叉查找树2

给出n，生成所有由1...n为节点组成的不同的二叉查找树

您在真实的面试中是否遇到过这个题？ 

Yes

样例

给出n = 3，生成所有5种不同形态的二叉查找树：

1         3     3       2    1 \       /     /       / \    \  3     2     1       1   3    2 /     /       \                \2     1         2                3

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @paramn n: An integer     * @return: A list of root     */    public ArrayList<TreeNode> generateTrees(int n) {        return generate(1, n);    }        private ArrayList<TreeNode> generate(int start, int end){        ArrayList<TreeNode> rst = new ArrayList<TreeNode>();           if(start > end){            rst.add(null);            return rst;        }        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;                    rst.add(root);                }            }        }            return rst;    }}