feat: update solutions to lc problem: No.0021 (#4038)

Author: djvelimir

solution/0000-0099/0021.Merge Two Sorted Lists/README.md

@@ -295,24 +295,19 @@ var mergeTwoLists = function (list1, list2) {
  */
 public class Solution {
     public ListNode MergeTwoLists(ListNode list1, ListNode list2) {
-        ListNode dummy = new ListNode();
-        ListNode cur = dummy;
-        while (list1 != null && list2 != null)
-        {
-            if (list1.val <= list2.val)
-            {
-                cur.next = list1;
-                list1 = list1.next;
-            }
-            else
-            {
-                cur.next = list2;
-                list2 = list2.next;
-            }
-            cur = cur.next;
+        if (list1 == null) {
+            return list2;
+        }
+        if (list2 == null) {
+            return list1;
+        }
+        if (list1.val <= list2.val) {
+            list1.next = MergeTwoLists(list1.next, list2);
+            return list1;
+        } else {
+            list2.next = MergeTwoLists(list1, list2.next);
+            return list2;
         }
-        cur.next = list1 == null ? list2 : list1;
-        return dummy.next;
     }
 }
 ```
@@ -332,23 +327,60 @@ public class Solution {
 # @param {ListNode} list2
 # @return {ListNode}
 def merge_two_lists(list1, list2)
-    dummy = ListNode.new()
-    cur = dummy
-    while list1 && list2
-        if list1.val <= list2.val
-            cur.next = list1
-            list1 = list1.next
-        else
-            cur.next = list2
-            list2 = list2.next
-        end
-        cur = cur.next
+    if list1.nil?
+        return list2
+    end
+    if list2.nil?
+        return list1
+    end
+    if list1.val <= list2.val
+        list1.next = merge_two_lists(list1.next, list2)
+        return list1
+    else
+        list2.next = merge_two_lists(list1, list2.next)
+        return list2
     end
-    cur.next = list1 || list2
-    dummy.next
 end
 ```
 
+#### PHP
+
+```php
+/**
+ * Definition for a singly-linked list.
+ * class ListNode {
+ *     public $val = 0;
+ *     public $next = null;
+ *     function __construct($val = 0, $next = null) {
+ *         $this->val = $val;
+ *         $this->next = $next;
+ *     }
+ * }
+ */
+class Solution {
+    /**
+     * @param ListNode $list1
+     * @param ListNode $list2
+     * @return ListNode
+     */
+    function mergeTwoLists($list1, $list2) {
+        if (is_null($list1)) {
+            return $list2;
+        }
+        if (is_null($list2)) {
+            return $list1;
+        }
+        if ($list1->val <= $list2->val) {
+            $list1->next = $this->mergeTwoLists($list1->next, $list2);
+            return $list1;
+        } else {
+            $list2->next = $this->mergeTwoLists($list1, $list2->next);
+            return $list2;
+        }
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->
@@ -603,6 +635,72 @@ var mergeTwoLists = function (list1, list2) {
 };
 ```
 
+#### C#
+
+```cs
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public int val;
+ *     public ListNode next;
+ *     public ListNode(int val=0, ListNode next=null) {
+ *         this.val = val;
+ *         this.next = next;
+ *     }
+ * }
+ */
+public class Solution {
+    public ListNode MergeTwoLists(ListNode list1, ListNode list2) {
+        ListNode dummy = new ListNode();
+        ListNode curr = dummy;
+        while (list1 != null && list2 != null) {
+            if (list1.val <= list2.val) {
+                curr.next = list1;
+                list1 = list1.next;
+            } else {
+                curr.next = list2;
+                list2 = list2.next;
+            }
+            curr = curr.next;
+        }
+        curr.next = list1 == null ? list2 : list1;
+        return dummy.next;
+    }
+}
+```
+
+#### Ruby
+
+```rb
+# Definition for singly-linked list.
+# class ListNode
+#     attr_accessor :val, :next
+#     def initialize(val = 0, _next = nil)
+#         @val = val
+#         @next = _next
+#     end
+# end
+# @param {ListNode} list1
+# @param {ListNode} list2
+# @return {ListNode}
+def merge_two_lists(list1, list2)
+    dummy = ListNode.new()
+    cur = dummy
+    while list1 && list2
+        if list1.val <= list2.val
+            cur.next = list1
+            list1 = list1.next
+        else
+            cur.next = list2
+            list2 = list2.next
+        end
+        cur = cur.next
+    end
+    cur.next = list1 || list2
+    dummy.next
+end
+```
+
 #### PHP
 
 ```php
@@ -616,18 +714,15 @@ var mergeTwoLists = function (list1, list2) {
 #         $this->next = $next;
 #     }
 # }
-
 class Solution {
     /**
      * @param ListNode $list1
      * @param ListNode $list2
      * @return ListNode
      */
-
     function mergeTwoLists($list1, $list2) {
         $dummy = new ListNode(0);
         $current = $dummy;
-
         while ($list1 != null && $list2 != null) {
             if ($list1->val <= $list2->val) {
                 $current->next = $list1;