Leetcode: Convert Sorted List to Binary Search Tree. java

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

每次取链表的中点作为root，然后递归

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
/**
 * Definition for binary tree
 * 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;
        if (head.next == null)
            return new TreeNode(head.val);
        //p为链表中点，从p处截断，右半头节点为q
        ListNode p = getMid(head);
        ListNode q = p.next;
        p.next = null;
        TreeNode root = new TreeNode(p.val);
        //删除p 新建一个虚拟的节点r r.next是左半段的头节点
        ListNode r = new ListNode(0);
        r.next = head;
        ListNode s = r;
        while (true) {
            if (s.next == p)
                break;
            s = s.next;
        }
        s.next = null;
        //递归
        root.left = sortedListToBST(r.next);
        root.right = sortedListToBST(q);
        return root;
    }
    
    public ListNode getMid(ListNode head) {
        if (head == null || head.next == null)
            return head;
        ListNode p = head;
        ListNode q = head;
        while (p.next != null && q.next != null && 
        q.next.next != null) {
            p = p.next;
            q = q.next.next;
        }
        return p;
    }
}

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