[LeetCode] 109. Convert Sorted List to Binary Sear

dongfangyiyu 發(fā)布于2019-08-16 13:42 / 2229人閱讀

Problem

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:

Solution

class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        return helper(head, null);
    }
    private TreeNode helper(ListNode head, ListNode tail) {
        if (head == tail) return null;
        ListNode fast = head, slow = head;
        while (fast != tail && fast.next != tail) {
            fast = fast.next.next;
            slow = slow.next;
        }
        TreeNode root = new TreeNode(slow.val);
        root.left = helper(head, slow);
        root.right = helper(slow.next, tail);
        return root;
    }
}

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