[LeetCode] Problem 109 - Convert Sorted List to Binary Search Tree

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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:

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2
3
4
5
     0
    / \
  -3   9
  /   /
-10  5

Code

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public class ListNode {
    int val;
    ListNode next = null;
    ListNode(int x) { val = x; }
}

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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// O(n) runtime, O(logn) stack space – Bottom-up recursion
private ListNode current;

public TreeNode sortedListToBST(ListNode head) {
    ListNode node = head;
    current = head;
    int length = 0;

    while (node != null) {
        length++;
        node = node.next;
    }

    return sortedListToBSTHelper(0, length - 1);
}

private TreeNode sortedListToBSTHelper(int start, int end) {
    if (start > end)
        return null;

    int mid = (start + end) >> 1;

    TreeNode left = sortedListToBSTHelper(start, mid - 1);

    TreeNode parent = new TreeNode(current.val);
    parent.left = left;
    current = current.next;

    TreeNode right = sortedListToBSTHelper(mid + 1, end);
    parent.right = right;

    return parent;
}