Leetcode-Unique Binary Search Trees II

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should 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? > read more on how binary tree is serialized on OJ.

Have you met this question in a real interview?

Solution:

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; left = null; right = null; }
 8  * }
 9  */
10 public class Solution {
11     public List<TreeNode> generateTrees(int n) {
12         List<TreeNode> list = generateTreesRecur(n,1,n);
13         return list;
14         
15     }
16 
17     public List<TreeNode> generateTreesRecur(int len, int s, int e){
18         List<TreeNode> treeList = new ArrayList<TreeNode>();
19         if (len==0){
20             treeList.add(null);
21             return treeList;
22         }
23 
24         if (len==1){
25             TreeNode newNode = new TreeNode(s);
26             treeList.add(newNode);
27             return treeList;
28         }
29 
30         for (int i=s;i<=e;i++){
31             int leftLen = i-s;
32             int rightLen = e-i;
33             List<TreeNode> leftList = generateTreesRecur(leftLen,s,i-1);
34             List<TreeNode> rightList = generateTreesRecur(rightLen,i+1,e);
35             for (int j=0;j<leftList.size();j++)
36                 for (int k=0;k<rightList.size();k++){
37                     TreeNode root = new TreeNode(i);
38                     root.left = leftList.get(j);
39                     root.right = rightList.get(k);
40                     treeList.add(root);
41                 }
42 
43         }
44 
45         return treeList;
46 
47    }
48 }