[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 node

cur

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.

 class Solution {
public:
ListNode* reverseKGroup(ListNode* head, int k) {
if (!hasKNodes(head, k)) return head;
ListNode* new_head = new ListNode(0);
new_head -> next = head;
ListNode* pre= new_head;
ListNode* cur = head;
while (hasKNodes(cur, k)) {
for (int i = 0; i < k - 1; i++) {
ListNode* temp = pre-> next;
pre-> next = cur -> next;
cur -> next = cur -> next -> next;
pre-> next -> next = temp;
}
pre= cur;
cur = pre-> next;
}
return new_head -> next;
}
private:
bool hasKNodes(ListNode* node, int k) {
int cnt = 0;
while (node) {
cnt++;
if (cnt >= k) return true;
node = node -> next;
}
return false;
}
};