Leetcode(109) Convert Sorted List to Binary Search Tree

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Description

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:

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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

解法

左右分别递归地建立子树即可，首先将链表转成list，其余方法与108题类似

具体代码如下：

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class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        List l = new ArrayList<Integer>();
        while (head != null) {
            l.add(head.val);
            head = head.next;
        }
        return helper(l, 0, l.size() - 1);
    }

    TreeNode helper(List nums, int start, int end) {
        if (start > end) {
            return null;
        }
        int val = (int) nums.get((start + end) / 2);
        TreeNode root = new TreeNode(val);
        root.left = helper(nums, start, (start + end) / 2 - 1);
        root.right = helper(nums, (start + end) / 2 + 1, end);
        return root;
    }
}