leetcode [109]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.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

題目大意：

將雙向鏈表轉化爲高度平衡的二叉樹。

解法：

遞歸的求解，每次使用快慢指針尋找找雙向鏈表的中點，並新建一個二叉樹節點，而後遞歸的求解樹的左右節點，函數中的tail參數是不能取到的節點。

java:

class Solution {
    private TreeNode toBST(ListNode head, ListNode tail){
        ListNode slow=head;
        ListNode fast=head;
        if (head == tail) return null;
        while (fast != tail && fast.next !=tail){
            fast = fast.next.next;
            slow=slow.next;
        }
        TreeNode node=new TreeNode(slow.val);
        node.left=toBST(head,slow);
        node.right=toBST(slow.next,tail);

        return node;
    }

    public TreeNode sortedListToBST(ListNode head) {
        if (head == null) return null;
        return toBST(head, null);
    }
}