LeetCode 109. Convert Sorted List to Binary Search Tree

109. Convert Sorted List to Binary Search Tree

Difficulty:: Medium

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

Language: Java
分治法，不使用dummyNode

/**
 * 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; }
 * }
 */
class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        if (head == null) {
            return null;
        }
        if (head.next == null) {
            return new TreeNode(head.val);
        }
        ListNode prev = null;
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        if (prev != null) {
            prev.next = null;
        }
        TreeNode center = new TreeNode(slow.val);
        // 需要注意slow和head重合的情况
        if (slow != head) {
            center.left = sortedListToBST(head);
        }
        center.right = sortedListToBST(slow.next);
        return center;
    }
}