109. Convert Sorted List to Binary Search Tree

Posted 2020-09-30

/**
 * 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) {
        if (head == tail) return null;
        ListNode fast = head;
        ListNode slow = head;
        while (fast != tail && fast.next != tail) {
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode cur = new TreeNode(slow.val);
        cur.left = helper(head, slow);
        cur.right = helper(slow.next, tail);
        return cur;
    }
}

考察后序建树和递归出口的设置: 

 return helper(head, null);

if (head == tail) return null;

虽然arraylist 是

return helper(nums, 0, nums.length - 1);

if (start > end) return null,

if (start == end) {
T  t = new  T(nums[end]);

root.left = null;

 root.right = null;

return root;