[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.

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;
    }
};