leetcode-109. Convert Sorted List to Binary Search Tree

leetcode-109. Convert Sorted List to Binary Search Tree

题目：

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

这题是个很有意思的题。这题一开始其实和上一题差不多，但是这里比较麻烦的是链表。如果按照上一题的做法需要遍历链表去定位，这就比较没麻烦了。所以这里需用使用快慢指针的方法去定位，这样就快的多了。

链表的题目是比较有意思的，比如哑节点，比如快慢指针，很多地方都很有技巧性，不过这些技巧都是可复制的。

/**
* 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) {
if(head==null) return null;
return helper(head,null);
}

private TreeNode helper(ListNode head, ListNode tail){
ListNode fast = head;
ListNode slow = head;
if(head == tail) return null;

while(fast!=tail&&fast.next!=tail){
fast = fast.next.next;
slow = slow.next;
}
TreeNode ret = new TreeNode(slow.val);
ret.left = helper(head,slow);
ret.right = helper(slow.next,tail);
return ret;
}
}