Leetcode 25. Reverse Nodes in k-Group 以每组k个结点进行链表反转(链表)

Input: k = 2, 1->2->3->4->5
Output: 2->1->4->3->5


Input: k = 3, 1->2->3->4->5
Output: 3->2->1->4->5

class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        if(head==nullptr || k==1){
            return head;
        }

        ListNode * cur = head;
        while(true){
            partialReverse(cur,k);
        
            for(int i=0;i<k;i++){
                cur=cur->next;
                if(cur==nullptr){
                    return head;
                }
            }
        }
    }
     
    // it's a closed interval
    void partialReverse(ListNode * start,int k){
        // 1->2
        // 1->2->3->4->5
        std::stack<int> vec;
        ListNode  * temp = start;
        for(int i=0;i<k;i++){
            vec.push(temp->val);
            temp = temp->next;
            if(i!=k-1 && temp==nullptr){
                return;
            }
        }
    
        for(int i=0;i<k;i++){
            int val = vec.top();
            vec.pop();
            start->val = val;
            start=start->next;
        }
    }
};