problems: add middle of the linked list

problems/middle-of-the-linked-list.cpp

@@ -0,0 +1,27 @@
+/**
+ * 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* middleNode(ListNode* head) {
+        auto slow = head;
+        auto fast = head ? head->next : nullptr;
+        
+        while (fast != nullptr) {
+            slow = slow->next;
+            fast = fast->next;
+            if (fast) {
+                fast = fast->next;
+            }
+        }
+        
+        return slow;
+    }
+};