Convert Sorted List to Binary Search Tree leetcode java

題目：

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

 

題解：

之前做過一道是從sorted array轉換到BinarySearchTree的，方法還是一樣二分法。但是構造樹的方法不是由頂至下了，是要由低至上的建立。

程式碼如下：

 

 1     static ListNode h;
 2  
 3     public TreeNode sortedListToBST(ListNode head) {
 4         if (head == null)
 5             return null;
 6             
 7         h = head;
 8         
 9         int len = 0;
10         ListNode temp = head;
11         while(temp != null){
12             len++;
13             temp = temp.next;
14         }   
15         return sortedListToBST(0, len - 1);
16     }
17  
18     public TreeNode sortedListToBST(int start, int end) {
19         if (start > end)
20             return null;
21         int mid = (start + end) / 2;
22         TreeNode left = sortedListToBST(start, mid - 1);
23         TreeNode root = new TreeNode(h.val);
24         root.left = left;
25         h = h.next;
26         TreeNode right = sortedListToBST(mid + 1, end);
27         root.right = right;
28  
29         return root;
30     }

 

Reference： http://www.programcreek.com/2013/01/leetcode-convert-sorted-list-to-binary-search-tree-java/