Unique Binary Search Trees II

GivenN, Generate all structurally uniqueBST's(Binary Search Trees) that store values 1...N.

For example,
GivenN= 3, your program shocould return all 5 unique BST's shown below.

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

Confused what"{1,#,2,3}"Means?

OJ's binary tree serialization:

The serialization of a binary tree follows a level order traversal, where '# 'signiies a path Terminator where no node exists below.

Here's an example:

   1  /  2   3    /   4         5

The above binary tree is serialized "{1,2,3,#,#,4,#,#,5}".

Answer

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; left = null; right = null; } * } */public class Solution {    public TreeNode copyNode(TreeNode node)    {        if(node==null)        {            return null;        }        TreeNode nodeCopy=new TreeNode(node.val);        List<TreeNode> listOriginal=new LinkedList<TreeNode>();        List<TreeNode> listCopy=new LinkedList<TreeNode>();        listOriginal.add(node);        listCopy.add(nodeCopy);        while(listOriginal.size()>0)        {            TreeNode original=listOriginal.get(0);            TreeNode copy=listCopy.get(0);            if(original.left!=null)            {                copy.left=new TreeNode(original.left.val);                listOriginal.add(original.left);                listCopy.add(copy.left);            }            if(original.right!=null)            {                copy.right=new TreeNode(original.right.val);                listOriginal.add(original.right);                listCopy.add(copy.right);            }            listOriginal.remove(0);            listCopy.remove(0);        }        return nodeCopy;    }    public List<TreeNode> generateTrees(int start,int end)    {        List<TreeNode> result=new LinkedList<TreeNode>();        TreeNode root=null;        if(start>end)        {            result.add(null);            return result;        }        for(int i=start;i<=end;i++)        {                        List<TreeNode> listLeft=generateTrees(start,i-1);            List<TreeNode> listRight=generateTrees(i+1,end);            for(TreeNode left:listLeft)            {                for(TreeNode right:listRight)                {                    root=new TreeNode(i);                    root.left=copyNode(left);                    root.right=copyNode(right);                    result.add(root);                }            }        }        return result;    }    public List<TreeNode> generateTrees(int n) {        if(n<0)        {            return new LinkedList<TreeNode>();        }        return generateTrees(1,n);    }}

Unique Binary Search Trees II