206. 反转链表

class Solution {

    public:

        ListNode* reverseList(ListNode* head) {

            ListNode* prev = nullptr;

            ListNode* current = head;

            while(current != nullptr){

                ListNode* tempNext = current->next;

                current->next = prev;

                prev = current;

                current = tempNext;

            }

            return prev;

        }

};

92. 反转链表 II

class Solution {

    public ListNode reverseBetween(ListNode head, int left, int right) {

        int count = 1;

        ListNode current = head;

        ListNode previous = null;

        ListNode tempNext = null;

        ListNode leftBreak = null;

        ListNode reverseTail = null;

        while(count <= right){

            tempNext = current.next;

            if(left == count){

                leftBreak = previous;

                reverseTail = current;

            } else if(left < count){

                current.next = previous;

            }

            previous = current;

            current = tempNext;

            count++;

        }

        reverseTail.next = current;

        if(leftBreak != null){

            leftBreak.next = previous;

         } else {

            return previous;

        }

        return head;

    }

}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        int count = 0;
        ListNode current = head;
        ListNode previous = null;
        ListNode newCurrent  = current;
        ListNode leftBreak = null,reverseTail = head, reverseHead = null;
        while(true){
            count ++;
            if(count == k){
                reverseHead = current;
                current = reverseTail;
                previous = null;
                while (previous != reverseHead){
                    newCurrent = current.next;
                    current.next = previous;
                    previous = current;
                    current = newCurrent;
                }
                if(leftBreak == null){
                    head = reverseHead;
                }else{
                    leftBreak.next = reverseHead;
                }
                leftBreak = reverseTail;
                reverseTail.next = current;
                reverseTail = current;
                count = 0;
            }else{
                current = current.next;
            }
            if(current == null){
                break;
            }

        }
        return head;
    }
}