[LeetCode] Reverse Nodes in k-Group

Well, since the head pointer may also be modified, we create a new_head that points to it to facilitate the reverse process.

For the example list 1 -> 2 -> 3 -> 4 -> 5 in the problem statement, it will become 0 -> 1 -> 2 -> 3 -> 4 -> 5 (we init new_head -> val to be 0). Then we set a pointer pre to new_head and another cur to head. Then we insert cur -> next after pre for k - 1 times if the current nodecur has at least k nodes after it (including itself). After reversing one k-group, we update pre to be cur and cur to be pre -> next to reverse the next k-group.

The code is as follows.

 1 class Solution { 
 2 public: 
 3     ListNode* reverseKGroup(ListNode* head, int k) {
 4         if (!hasKNodes(head, k)) return head;
 5         ListNode* new_head = new ListNode(0);
 6         new_head -> next = head;
 7         ListNode* pre = new_head;
 8         ListNode* cur = head;
 9         while (hasKNodes(cur, k)) {
10             for (int i = 0; i < k - 1; i++) {
11                 ListNode* temp = pre -> next;
12                 pre -> next = cur -> next;
13                 cur -> next = cur -> next -> next;
14                 pre -> next -> next = temp; 
15             }
16             pre = cur;
17             cur = pre -> next;
18         }
19         return new_head -> next;
20     }
21 private:
22     bool hasKNodes(ListNode* node, int k) {
23         int cnt = 0;
24         while (node) {
25             cnt++;
26             if (cnt >= k) return true;
27             node = node -> next;
28         }
29         return false;
30     }
31 };