Create 143-Reorder-List.js

javascript/143-Reorder-List.js

@@ -0,0 +1,50 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ * this.val = (val===undefined ? 0 : val)
+ * this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {void} Do not return anything, modify head in-place instead.
+ */
+var reorderList = function(head) {
+ if (!head) { return };
+
+ let slow = head;
+ let fast = head;
+
+
+ // finding the middle of the linked list using 2 pters
+ while (fast && fast.next) {
+ slow = slow.next;
+ fast = fast.next.next;
+ }
+
+ // reverse the second part of the list starting at slow
+ let prev = null
+ let curr = slow; 
+ while (curr) {
+ let next = curr.next;
+ curr.next = prev;
+ prev = curr;
+ curr = next;
+ } // here prev is the head 
+
+ // merge two sorted lists (first one starts at head, second at prev)
+ let first = head;
+ let second = prev;
+
+ while(second.next) {
+ temp = first.next;
+ first.next = second;
+ first = temp;
+
+ temp = second.next;
+ second.next = first;
+ second = temp;
+ }
+
+
+};