[leetcode]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.

算法思路：

与[leetcode]Convert Sorted Array to Binary Search Tree很相似，不过一个是array，一个是list，无本质区别。

三步走：

找中点（快慢指针）

断链表（需要找慢指针的前驱）

分治。

代码如下：

public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null) return null;
        if(head.next == null) return new TreeNode(head.val);
        ListNode hhead = new ListNode(0);
        hhead.next = head;
        ListNode fast = hhead, slow= hhead;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode root = new TreeNode(slow.val);
        ListNode right = slow.next;
        ListNode pre = hhead;
        while(pre != null && pre.next != slow){
            pre = pre.next;
        }
        pre.next = null;
        ListNode left = hhead.next;
        root.left = sortedListToBST(left);
        root.right = sortedListToBST(right);
        return root;
    }
}