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

Author: kamyu104

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

@@ -0,0 +1,30 @@
+// Time:  O(m + n)
+// Space: O(1)
+
+/**
+ * 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* mergeInBetween(ListNode* list1, int a, int b, ListNode* list2) {
+        ListNode* prev_first = nullptr, *last = list1;
+        for (int i = 0; i < b; ++i) {
+            if (i == a - 1) {
+                prev_first = last;
+            }
+            last = last->next;
+        }
+        prev_first->next = list2;
+        for (; list2->next; list2 = list2->next);
+        list2->next = last->next;
+        last->next = nullptr;
+        return list1;
+    }
+};