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


 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 ArrayList<TreeNode> generateTrees(int n) {
12         // Start typing your Java solution below
13         // DO NOT write main() function
14         return generateTrees(1,n);
15     }
16     public ArrayList<TreeNode> generateTrees(int a, int b){
17         ArrayList<TreeNode> res = new ArrayList<TreeNode>();
18         if(a>b) res.add(null);   
19         for(int i=a;i<=b;i++){
20             ArrayList<TreeNode> temp1 = generateTrees(a,i-1);
21             ArrayList<TreeNode> temp2 = generateTrees(i+1,b);
22             for(TreeNode n:temp1)
23                 for(TreeNode m:temp2){
24                     TreeNode temp= new TreeNode(i);
25                     temp.left=n;
26                     temp.right=m;
27                     res.add(temp);
28                 }
29         }
30         return res;
31     }
32 }

 

posted on 2013-09-26 02:44  Step-BY-Step  阅读(194)  评论(0)  编辑  收藏  举报

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