Create 143-Reorder-List.js

Author: veerbia

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;
+    }
+    
+    
+};