LeetCode Convert Sorted List to Binary Search Tree 解题报告

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

从给定的有序链表生成平衡二叉树。

解题思路：最容易想到的就是利用数组生成二叉树的方法，找到中间节点作为二叉树的root节点，然后分别对左右链表递归调用分别生成左子树和右子树。时间复杂度O(N*lgN)

AC代码：

public class Solution {
    ListNode getLeftNodeFromList(ListNode head) {
        ListNode next = head;
        ListNode current = head;
        ListNode pre = head;

        while(next!=null) {
            next = next.next;
            if(next==null) {
                break;
            }
            next = next.next;
            if(next==null) {
                break;
            }
            pre = head;
            head = head.next;
        }
        return pre;
    }

    public TreeNode sortedListToBST(ListNode head) {
        if(head==null) {
            return null;
        }
        if(head.next==null) {
            return new TreeNode(head.val);
        }

        ListNode left = getLeftNodeFromList(head);
        ListNode mid = left.next;
        TreeNode root = new TreeNode(mid.val);
        left.next     = null;
        root.left     = sortedListToBST(head);
        root.right    = sortedListToBST(mid.next);
        return root;
    }
}