leetcode+翻转链表

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struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode *reverseKGroup(ListNode *head, int k) {
        ListNode *dummy = new ListNode(-1), *pre = dummy, *cur = pre;
        dummy->next = head;
        int num = 0;
        while(cur = cur->next) ++num;
        while(num>=k){
            cur = pre->next;
            //在一段链表中间一段子链表翻转的经典模版
            for(int i=1; i<k; i++){//pre->a->b->c 先pre->b->a->c 再pre->c->b->a 把元素依次往前插
                ListNode* t = cur->next;
                cur->next = t->next;
                t->next = pre->next;
                pre->next = t;;
            }
            pre = cur;//刚开始最前面的cur也就是下一阶段的pre， 然后pre->next就是下一个的cur了
            num -= k;
        }
        return dummy->next;
    }
};