Update and rename reorderList.cpp to reorder-list.cpp

Author: kamyu104

C++/reorder-list.cpp

@@ -0,0 +1,70 @@
+// Time:  O(n)
+// Space: O(1)
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+    public:
+        void reorderList(ListNode *head) {
+        if (!head) {
+            return;
+        }
+
+        auto slow = head, fast = head;
+
+        while (fast->next && fast->next->next) {
+            slow = slow->next;
+            fast = fast->next->next;
+        }
+
+        // Split into two lists.
+        auto tmp = slow->next;
+        slow->next = nullptr;
+        slow = tmp;
+
+        merge(head, reverse(slow));
+    }
+
+private:
+    ListNode *reverse(ListNode *head) {
+        ListNode dummy{0};
+
+        while (head) {
+            auto tmp = head->next;
+            head->next = dummy.next;
+            dummy.next = head;
+            head = tmp;
+        }
+
+        return dummy.next;
+    }
+
+    ListNode *merge(ListNode *list1, ListNode *list2) {
+        ListNode dummy{0};
+        auto ptr = &dummy;
+
+        while (list1 && list2) {
+            auto tmp = list1->next;
+
+            ptr->next = list1;
+            ptr = ptr->next;
+            ptr->next = list2;
+            ptr = ptr->next;
+
+            list1 = tmp;
+            list2 = list2->next;
+        }
+
+        if (list1) {
+            ptr->next = list1;
+        }
+
+        return dummy.next;
+    }
+};