leetcode 109 Convert Sorted List to Binary Search Tree ----- java

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

 

和上一题类似，把数组换成链表，所以可以两种做法：

1、把链表换成数组，然后用上一题的方法，这样会比较慢。

2、每次找到中间的点，作为节点，然后递归，其实原理还是二分查找。

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        List list = new ArrayList<Integer>();
        if( head == null)
            return null;
        return helper(head,null);
    }
    public TreeNode helper(ListNode head,ListNode target){

        if( head == target )
            return null;
        ListNode node1 = head;
        ListNode node2 = head;

        while( node2 != target && node2.next != target){
            node1 = node1.next;
            node2 = node2.next.next;
        }
        TreeNode node = new TreeNode(node1.val);

        node.left = helper(head,node1);
        node.right = helper(node1.next,target);

        return node;

        }
}

第一种：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        List list = new ArrayList<Integer>();
        if( head == null)
            return null;
        while( head != null ){
            list.add(head.val);
            head = head.next;
        }
        int[] nums = new int[list.size()];
        for( int i = 0;i<list.size();i++)
            nums[i] = (int) list.get(i);
        return sortedArrayToBST(nums);
    }
    public TreeNode sortedArrayToBST(int[] nums) {
            int len = nums.length;
            return helper(nums,0,(len-1)/2,len-1);

        }

        public TreeNode helper(int[] nums,int start,int mid,int end){

            if( start > end )
                return null;
            TreeNode node = new TreeNode(nums[mid]);

            node.left = helper(nums,start,(mid+start-1)/2,mid-1);

            node.right = helper(nums,mid+1,(end+mid+1)/2,end);

            return node;

        }
}