64_Reverese_Nodes_in_K-Group

/**
 * 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) {}
 * };
 */
int lengthOfLinkedList(ListNode *head){
    ListNode *ptr=head;
    int k=0;
    while(ptr->next!=NULL){
        ptr=ptr->next;
        k++;
    }
    return k+1;
}
class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        if(head == NULL||head->next == NULL) return head;
    
    int length = lengthOfLinkedList(head);
    
    ListNode* dummyHead = new ListNode(0);
    dummyHead->next = head;
    
    ListNode* pre = dummyHead;
    ListNode* cur;
    ListNode* nex;
    
    while(length >= k) {
        cur = pre->next;
        nex = cur->next;
        for(int i=1;i<k;i++) {
            cur->next = nex->next;
            nex->next = pre->next;
            pre->next = nex;
            nex = cur->next;
        }
        pre = cur;
        length -= k;
    }
    return dummyHead->next;
    }
};