【LeetCode】109.Convert Sorted List to Binary Search Tree(Medium)解题报告

【LeetCode】109.Convert Sorted List to Binary Search Tree(Medium)解题报告

题目地址：https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/description/
题目描述：

  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

Solution:

/**
 * 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; }
 * }
 链表转换成bst
 time : O(n)
 space : O(n)
 */
class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null) return null;
        return toBST(head,null);
    }
    public TreeNode toBST(ListNode head,ListNode tail){
        if(head == tail) return null;
        ListNode slow = head;
        ListNode fast = head;
        while(fast != tail && fast.next != tail){
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode root = new TreeNode(slow.val);
        root.left = toBST(head,slow);
        root.right = toBST(slow.next,tail);
        return root;
    }
}

Date：2018年3月29日