Convert Sorted Array to Binary Search Tree

 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; }
 8  * }
 9  */
10 public class Solution {
11     public TreeNode root ;
12 
13     public TreeNode sortedArrayToBST(int[] num) {
14         if(num.length == 0)
15             return root;
16         if(1 == num.length){
17             return new TreeNode(num[0]);
18         }
19         int middle = num.length / 2;
20         root = new TreeNode(num[middle]);
21         
22         createBST(num, 0, middle - 1);
23         createBST(num, middle + 1, num.length - 1);
24         return root;
25     }
26     
27     /**
28      * 根据num数组，创建一棵二叉查找树
29      * @param num
30      * @param start
31      * @param end
32      */
33     private void createBST(int num[], int start, int end){
34         int middle = 0;
35         if(start <= end && start >= 0 && end <num.length){
36             middle = (start + end) / 2;
37             
38             insertNode(root, num[middle]);
39             
40             createBST(num, start, middle - 1);
41             createBST(num, middle + 1, end);
42         }
43     }
44     
45     /**
46      * 向root所指的BST二叉查找树中插入value
47      * @param root
48      * @param value
49      */
50     private void insertNode(TreeNode root, int value){        
51         if(value > root.val){                    //比根节点大,在右子树插入
52             if(root.right == null){
53                 root.right = new TreeNode(value);
54             }else{
55                 root = root.right;
56                 insertNode(root, value);
57             }
58         }
59         else{
60             if(root.left == null){
61                 root.left = new TreeNode(value);
62             }else{
63                 root = root.left;
64                 insertNode(root, value);            //比根节点小的插入左子树
65             }
66         }
67     }
68     
69 //    /**
70 //     * 先序遍历
71 //     * @param root
72 //     */
73 //    public void preTravel(TreeNode root){
74 //        if(root != null){
75 //            System.out.print(root.val + " ");
76 //            preTravel(root.left);
77 //            preTravel(root.right);
78 //        }
79 //    }
80 }