Create 143.Reorder-List_v2.cpp

1. Use fast and slow pointers;
2. More elegant revert and insertion.

Linked_List/143.Reorder-List/143.Reorder-List_v2.cpp

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+/**
+ * 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:
+    void reorderList(ListNode* head) {
+        ListNode *slow = head, *fast = head;
+        for(; fast && fast->next; slow = slow->next, fast = fast->next->next);
+        ListNode *head1 = head;
+        ListNode *head2 = slow->next;
+        slow->next = nullptr;
+
+        // reverse list2
+        ListNode *nextNode = nullptr;
+        for(ListNode *cur = head2; cur != nullptr;) {
+            ListNode *curNext = cur->next;
+            cur->next = nextNode;
+            nextNode = cur;
+            cur = curNext;
+        }
+        head2 = nextNode;
+
+        // traverse list1 to insert
+        for(ListNode *ptr1 = head1, *ptr2 = head2; ptr1 && ptr2;) {
+            ListNode *ptr1Next = ptr1->next, *ptr2Next = ptr2->next;
+            ptr1->next = ptr2;
+            ptr2->next = ptr1Next;
+            ptr1 = ptr1Next;
+            ptr2 = ptr2Next;
+        }
+    }
+};