Added merge two sorted list solution

Author: nhistory

merge-two-sorted-lists/nhistory.js

@@ -0,0 +1,44 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} list1
+ * @param {ListNode} list2
+ * @return {ListNode}
+ */
+var mergeTwoLists = function (list1, list2) {
+  // Make sure both or one of the list is null
+  if (!list1 && !list2) return null;
+  if (!list1) return list2;
+  if (!list2) return list1;
+
+  // Create dummy listNode
+  let dummy = new ListNode(0);
+  let head = dummy;
+  // Make head of dummy list by using smaller node from list1 and list2
+  while (list1 && list2) {
+    if (list1.val <= list2.val) {
+      dummy.next = list1;
+      list1 = list1.next;
+    } else {
+      dummy.next = list2;
+      list2 = list2.next;
+    }
+    // After choosing with dummy list, head should be moved next
+    dummy = dummy.next;
+  }
+  // Iterate until both of list head is equal to null
+  if (list1 !== null) {
+    dummy.next = list1;
+  } else {
+    dummy.next = list2;
+  }
+  return head.next;
+};
+
+// TC: O(m+n)
+// SC: O(1)