LeetCode-95-不同的二叉搜索树 II

示例：

输入：3
输出：
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]

解释：
以上的输出对应以下 5 种不同结构的二叉搜索树：

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

 1 public class LeetCode_95 {
 2     
 3      public class TreeNode {
 4           int val;
 5           TreeNode left;
 6           TreeNode right;
 7           TreeNode() {}
 8           TreeNode(int val) { this.val = val; }
 9           TreeNode(int val, TreeNode left, TreeNode right) {
10               this.val = val;
11               this.left = left;
12               this.right = right;
13           }
14       }
15           
16     public List<TreeNode> generateTrees(int n) {
17         
18         List<TreeNode> re = new ArrayList<LeetCode_95.TreeNode>();
19         
20         if(n==0) return re;
21         
22         re = generateTrees(1,n);
23         
24         return re;
25     }
26     
27     
28     public List<TreeNode> generateTrees(int star,int end){
29         
30         List<TreeNode> re = new ArrayList<LeetCode_95.TreeNode>();
31         
32         if(star>end) re.add(null);
33         
34         for(int i=star;i<=end;i++){
35             
36             List<TreeNode> leftTrees = generateTrees(star, i-1); // 获取所有左子树
37             
38             List<TreeNode> rightTrees = generateTrees(i+1, end); // 获取所有右子树
39             
40             for (TreeNode left : leftTrees) {     // 遍历左右子树，将其拼接在跟节点上
41                 for (TreeNode right : rightTrees) {
42                     TreeNode currTree = new TreeNode(i);
43                     currTree.left = left;
44                     currTree.right = right;
45                     re.add(currTree);
46                 }
47             }
48 
49 
50         }
51               
52         return re;
53     }
54     
55 }