cpp: add «1669. Merge In Between Linked Lists»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cpp/merge-in-between-linked-lists.cpp

@@ -0,0 +1,34 @@
+#ifdef _MF_TEST
+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) {}
+};
+#endif
+
+class Solution {
+  public:
+    ListNode *mergeInBetween(ListNode *list1, int a, int b, ListNode *list2) {
+        auto left = list1;
+        for (int i = 1; i < a; ++i) {
+            left = left->next;
+        }
+
+        auto right = left;
+        for (int i = a; i < b + 2; ++i) {
+            right = right->next;
+        }
+
+        left->next = list2;
+
+        auto last_mid = list2;
+        while (last_mid->next != nullptr) {
+            last_mid = last_mid->next;
+        }
+        last_mid->next = right;
+
+        return list1;
+    }
+};