Revise ruby solution 021 and add 023

Author: MarkKuang1991

solution/021.Merge Two Sorted Lists/Solution.rb

@@ -11,29 +11,15 @@
 # @param {ListNode} l2
 # @return {ListNode}
 def merge_two_lists(l1, l2)
-  return l1 if l2.nil?
-  return l2 if l1.nil?
-
-  head = ListNode.new(0)
-  temp = head
-
-  until l1.nil? && l2.nil?
-    if l1.nil?
-      head.next = l2
-      break
-    elsif l2.nil?
-      head.next = l1
-      break
-    elsif l1.val < l2.val
-      head.next = ListNode.new(l1.val)
-      head = head.next
-      l1 = l1.next
-    else
-      head.next = ListNode.new(l2.val)
-      head = head.next
-      l2 = l2.next
-    end
+  if l1.nil?
+    l2
+  elsif l2.nil?
+    l1
+  elsif l1.val < l2.val
+    l1.next = merge_two_lists(l1.next, l2)
+    l1
+  else
+    l2.next = merge_two_lists(l1, l2.next)
+    l2
   end
-  temp.next
 end
-

solution/023.Merge k Sorted Lists/Solution.rb

@@ -0,0 +1,39 @@
+# Definition for singly-linked list.
+# class ListNode
+#     attr_accessor :val, :next
+#     def initialize(val)
+#         @val = val
+#         @next = nil
+#     end
+# end
+
+# @param {ListNode[]} lists
+# @return {ListNode}
+def merge_k_lists(lists)
+  if lists.nil?
+    nil
+  elsif lists.length == 1
+    lists[0]
+  else
+    cur_list = lists[0]
+    for i in 1..lists.length - 1
+      cur_list = merge_2_lists(cur_list, lists[i])
+    end
+    cur_list
+  end
+
+end
+
+def merge_2_lists(list_a, list_b)
+  if list_a.nil?
+    list_b
+  elsif list_b.nil?
+    list_a
+  elsif list_a.val < list_b.val
+    list_a.next = merge_2_lists(list_a.next, list_b)
+    list_a
+  else
+    list_b.next = merge_2_lists(list_a, list_b.next)
+    list_b
+  end
+end