Convert Sorted List to Binary Search Tree

Convert Sorted List to Binary Search Tree

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

这道题我开始超时了，后面想了个投机取巧的办法。想转换成数组，然后使用上面一道题的方法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode root ;
    public int nums[];
    
    public TreeNode sortedListToBST(ListNode head) {       
        ListNode temp = head;
        int length = 0;
        while(temp != null){
            length ++;
            temp = temp.next;                    
        }
        nums = new int[length];
        temp = head;
        for(int i = 0; i < nums.length; i++){
            nums[i] = temp.val;
            temp = temp.next;
        }
        return sortedArrayToBST(nums);
    }

    public TreeNode sortedArrayToBST(int[] num) {
        if(num.length == 0)
            return root;
        if(1 == num.length){
            return new TreeNode(num[0]);
        }
        int middle = num.length / 2;
        root = new TreeNode(num[middle]);
        
        createBST(num, 0, middle - 1);
        createBST(num, middle + 1, num.length - 1);
        return root;
    }
    
    /**
     * 根据num数组，创建一棵二叉查找树
     * @param num
     * @param start
     * @param end
     */
    private void createBST(int num[], int start, int end){
        int middle = 0;
        if(start <= end && start >= 0 && end <num.length){
            middle = (start + end) / 2;
            
            insertNode(root, num[middle]);
            
            createBST(num, start, middle - 1);
            createBST(num, middle + 1, end);
        }
    }
    
    /**
     * 向root所指的BST二叉查找树中插入value
     * @param root
     * @param value
     */
    private void insertNode(TreeNode root, int value){        
        if(value > root.val){                    //比根节点大,在右子树插入
            if(root.right == null){
                root.right = new TreeNode(value);
            }else{
                root = root.right;
                insertNode(root, value);
            }
        }
        else{
            if(root.left == null){
                root.left = new TreeNode(value);
            }else{
                root = root.left;
                insertNode(root, value);            //比根节点小的插入左子树
            }
        }
    }
    
    /**
     * 先序遍历
     * @param root
     */
    public void preTravel(TreeNode root){
        if(root != null){
//            System.out.print(root.val + " ");
            preTravel(root.left);
            preTravel(root.right);
        }
    }
}