add first solve code

Author: colorbox

21/1.cpp

@@ -0,0 +1,52 @@
+/**
+ * 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;
+
+        ListNode *ret;
+        if(list1->val < list2->val){
+            ret = list1;
+            list1 = list1->next;
+        } else{
+            ret = list2;
+            list2 = list2->next;
+        }
+        ListNode *current = ret;
+
+        cout << current->val << endl;
+
+        while(list1 != nullptr || list2 != nullptr){
+            if(list1 == nullptr){
+                current->next = list2;
+                return ret;
+            }
+
+            if(list2 == nullptr){
+                current->next = list1;
+                return ret;
+            }
+
+            if(list1->val < list2->val){
+                current->next = list1;
+                list1 = list1->next;
+            }else{
+                current->next = list2;
+                list2 = list2->next;
+            }
+            current = current->next;
+        }
+
+        return ret;
+    }
+};