Convert Sorted List to Binary Search Tree

  1 /**
  2  * Definition for singly-linked list.
  3  * public class ListNode {
  4  *     int val;
  5  *     ListNode next;
  6  *     ListNode(int x) { val = x; next = null; }
  7  * }
  8  */
  9 /**
 10  * Definition for binary tree
 11  * public class TreeNode {
 12  *     int val;
 13  *     TreeNode left;
 14  *     TreeNode right;
 15  *     TreeNode(int x) { val = x; }
 16  * }
 17  */
 18 public class Solution {
 19     public TreeNode root ;
 20     public int nums[];
 21     
 22     public TreeNode sortedListToBST(ListNode head) {       
 23         ListNode temp = head;
 24         int length = 0;
 25         while(temp != null){
 26             length ++;
 27             temp = temp.next;                    
 28         }
 29         nums = new int[length];
 30         temp = head;
 31         for(int i = 0; i < nums.length; i++){
 32             nums[i] = temp.val;
 33             temp = temp.next;
 34         }
 35         return sortedArrayToBST(nums);
 36     }
 37 
 38     public TreeNode sortedArrayToBST(int[] num) {
 39         if(num.length == 0)
 40             return root;
 41         if(1 == num.length){
 42             return new TreeNode(num[0]);
 43         }
 44         int middle = num.length / 2;
 45         root = new TreeNode(num[middle]);
 46         
 47         createBST(num, 0, middle - 1);
 48         createBST(num, middle + 1, num.length - 1);
 49         return root;
 50     }
 51     
 52     /**
 53      * 根据num数组，创建一棵二叉查找树
 54      * @param num
 55      * @param start
 56      * @param end
 57      */
 58     private void createBST(int num[], int start, int end){
 59         int middle = 0;
 60         if(start <= end && start >= 0 && end <num.length){
 61             middle = (start + end) / 2;
 62             
 63             insertNode(root, num[middle]);
 64             
 65             createBST(num, start, middle - 1);
 66             createBST(num, middle + 1, end);
 67         }
 68     }
 69     
 70     /**
 71      * 向root所指的BST二叉查找树中插入value
 72      * @param root
 73      * @param value
 74      */
 75     private void insertNode(TreeNode root, int value){        
 76         if(value > root.val){                    //比根节点大,在右子树插入
 77             if(root.right == null){
 78                 root.right = new TreeNode(value);
 79             }else{
 80                 root = root.right;
 81                 insertNode(root, value);
 82             }
 83         }
 84         else{
 85             if(root.left == null){
 86                 root.left = new TreeNode(value);
 87             }else{
 88                 root = root.left;
 89                 insertNode(root, value);            //比根节点小的插入左子树
 90             }
 91         }
 92     }
 93     
 94     /**
 95      * 先序遍历
 96      * @param root
 97      */
 98     public void preTravel(TreeNode root){
 99         if(root != null){
100 //            System.out.print(root.val + " ");
101             preTravel(root.left);
102             preTravel(root.right);
103         }
104     }
105 }