Add 2nd and 3rd codes

Author: colorbox

21/2.cpp

@@ -0,0 +1,51 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
+        if(list1 == nullptr)return list2;
+        if(list2 == nullptr)return list1;
+
+        // always list1 <= list2
+        if(list1->val > list2->val){
+            swap(list1, list2);
+        }
+
+        ListNode* ret = list1;
+        list1 = list1->next;
+
+        ListNode *current = ret;
+
+        while(list1 != nullptr && list2 != nullptr){
+            // always list1 <= list2
+            if(list1->val > list2->val){
+                swap(list1, list2);
+            }
+
+            current->next = list1;
+            list1 = list1->next;
+
+            current = current->next;
+        }
+
+        if(list1 == nullptr){
+            current->next = list2;
+            return ret;
+        }
+
+        if(list2 == nullptr){
+            current->next = list1;
+            return ret;
+        }
+
+        return ret;
+    }
+};

21/3.cpp

@@ -0,0 +1,36 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
+        ListNode ret = ListNode();
+        ListNode *current = &ret;
+
+        while(list1 != nullptr && list2 != nullptr){
+            if(list1->val > list2->val){
+                swap(list1, list2);
+            }
+
+            current->next = list1;
+            current = current->next;
+            list1 = list1->next;
+        }
+
+        if(list1 == nullptr){
+            current->next = list2;
+        }
+        if(list2 == nullptr){
+            current->next = list1;
+        }
+
+        return ret.next;
+    }
+};