[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 heightbalanced BST.

思路：二分法；

/**
 * 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; }
 * }
 */
class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if (head==null)                              //如果一开始传进来的是空链表，返回null
            return null;
        if (head.next==null)                      //递归终止条件
            return new TreeNode(head.val);
        ListNode p = head,q = head,m = head;    //快慢指针找中间值
        while (q!=null&&q.next!=null){
            q = q.next.next;
            p = p.next;
        }
        q = p.next;
        TreeNode root = new TreeNode (p.val);
        while (m.next!=p)                                    
            m = m.next;                   
        m.next = null;
        root.left = sortedListToBST(head);            //分治
        root.right = sortedListToBST(q);
        return root;
        
    }
}