Create 2130-maximum-twin-sum-of-a-linked-list.js

Added solution for maximum-twin-sum-of-a-linked-list in JS.

Author: aadil42

Diff for: javascript/2130-maximum-twin-sum-of-a-linked-list.js

@@ -0,0 +1,59 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+
+/**
+ * Linear Time 
+ * Time O(n) | Space O(1)
+ * @param {ListNode} head
+ * @return {number}
+ */
+const llLength = (head) => {
+  let count = 0;
+  while (head) {
+    head = head.next;
+    count++;
+  }
+  return count;
+};
+
+const reverseLL = (head, len) => {
+  let count = 0;
+  let temp = null;
+  while (count < len) {
+    const next = head.next;
+    head.next = temp;
+    temp = head;
+    head = next;
+    count++;
+  }
+  return temp;
+};
+
+var pairSum = function (head) {
+  const mid = llLength(head) / 2;
+
+  // get the right starting pointer
+  let count = 0;
+  let rightPointer = head;
+  while (count < mid) {
+    rightPointer = rightPointer.next;
+    count++;
+  }
+
+  // reverse the left portion of the array
+  leftPointer = reverseLL(head, mid);
+
+  let max = 0;
+  while (leftPointer && rightPointer) {
+    max = Math.max(leftPointer.val + rightPointer.val, max);
+    leftPointer = leftPointer.next;
+    rightPointer = rightPointer.next;
+  }
+
+  return max;
+};