Create 109. Convert Sorted List to Binary Search Tree.java

Author: futurus

109. Convert Sorted List to Binary Search Tree.java

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+// https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree
+
+// NB: an extension of prob 108. Use slow/fast linked-list traversal to find mid element
+
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+public class Solution {
+    public TreeNode helper(ListNode head, ListNode tail) {
+        if (head == tail) return null;
+        ListNode runner = head, runnerX2 = head;
+
+        while (runnerX2.next != tail && runnerX2.next.next != tail) {
+            runner = runner.next;
+            runnerX2 = runnerX2.next.next;
+        }
+
+        TreeNode root = new TreeNode(runner.val);
+        root.left = helper(head, runner);
+        root.right = helper(runner.next, tail);
+        
+        return root;
+    }
+    
+    public TreeNode sortedListToBST(ListNode head) {
+        if (head == null) return null;
+        return helper(head, null);
+    }
+}