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?

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
      
        generateTrees(int n) {12         List
       
         list = generateTreesRecur(n,1,n);13         return list;14         15     }16 17     public List
        
          generateTreesRecur(int len, int s, int e){18         List
         
           treeList = new ArrayList
          
           ();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
           
             leftList = generateTreesRecur(leftLen,s,i-1);34 List
            
              rightList = generateTreesRecur(rightLen,i+1,e);35 for (int j=0;j