109. Convert Sorted List to Binary Search Tree

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

 

class Solution {
    private ListNode node;

    public TreeNode sortedListToBST(ListNode head) {

          if(head == null){
          return null;
        }
    
        int size = 0;
        ListNode runner = head;
        node = head;
    
        while(runner != null){
            runner = runner.next;
            size ++;
        }
    
        return inorderHelper(0, size - 1);
    }
    
    public TreeNode inorderHelper(int start, int end){
        if(start > end){
            return null;
        }
    
        int mid = start + (end - start) / 2;
        TreeNode left = inorderHelper(start, mid - 1);
    
        TreeNode treenode = new TreeNode(node.val);
        treenode.left = left;
        node = node.next;

        TreeNode right = inorderHelper(mid + 1, end);
        treenode.right = right;
    
        return treenode;
    }
}