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LeetCode solutions

109. Convert Sorted List to Binary Search Tree

My accepted JavaScript solution to LeetCode problem 109, Convert Sorted List to Binary Search Tree, running in 4ms.

  • Difficulty: Medium
  • JavaScript
  • Runtime 4ms
  • Memory 60.9MB

Read the problem on LeetCode View on GitHub

The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.

JavaScript

Accepted on LeetCode — runtime 4ms, memory 60.9MB, accepted 2025-12-24.

javascript
/**
 * @param {ListNode} head
 * @return {TreeNode}
 */
var sortedListToBST = function(head) {
    if (!head) return null;
    if (!head.next) return new TreeNode(head.val);
    
    // Find middle using slow/fast pointers
    let slow = head, fast = head, prev = null;
    
    while (fast && fast.next) {
        prev = slow;
        slow = slow.next;
        fast = fast.next.next;
    }
    
    // Disconnect left half
    if (prev) prev.next = null;
    
    // slow is the middle node
    const root = new TreeNode(slow.val);
    
    // Build subtrees
    root.left = sortedListToBST(prev ? head : null);
    root.right = sortedListToBST(slow.next);
    
    return root;
};

Source