【leetcode刷题笔记】Unique Binary Search Trees II

   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     private ArrayList<TreeNode> generate(int start,int end){
12         ArrayList<TreeNode> answer = new ArrayList<TreeNode>();
13         if(start > end){
14             answer.add(null);
15             return answer;
16         }
17         
18         //for every node from start to right,make it as tree root and recursively build its left and right child tree
19         for(int i = start; i <= end;i++){
20             ArrayList<TreeNode> left = generate(start, i-1);
21             ArrayList<TreeNode> right = generate(i+1, end);
22             for(TreeNode l:left){
23                 for(TreeNode r:right){
24                     TreeNode root = new TreeNode(i);
25                     root.left = l;
26                     root.right = r;
27                     answer.add(root);
28                 }
29             }
30             
31         }
32         
33         return answer;
34         
35     }
36     public List<TreeNode> generateTrees(int n) {
37         return generate(1,n);
38     }
39 }