天题之reverse list

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最后那个reverse函数写的比较费劲，主要是pre.next 和last.next必须指向正确位置

ref 解释 http://www.cnblogs.com/lichen782/p/leetcode_Reverse_Nodes_in_kGroup.html

ref 代码 http://www.cnblogs.com/springfor/p/3864530.html

public class Solution {    public ListNode reverseKGroup(ListNode head, int k) {        if(head==null || k<2) return head;        ListNode dum = new ListNode(-1);        dum.next = head;        ListNode pre = dum, cur = head;        int cnt=0;                while(cur!=null){            cnt++;            ListNode next = cur.next;            if(cnt==k){                  pre = reverse(pre, next);                cnt=0;             }            cur = next;                    }        return dum.next;    }        public   ListNode reverse(ListNode pre, ListNode next){ //pre  1->2->3 next   to  1<-2<-3, return 1        ListNode last =  pre.next ;        ListNode cur = last.next;        while(cur!=next){          <strong><span style="color:#ff0000;background-color: rgb(255, 255, 204);">  // ListNode tmp = cur.next; 这里是错误的关键，必须要把last指向正确的下一个位置          last.next = cur.next;            cur.next = pre.next;            pre.next = cur;            // cur = tmp;          cur =  last.next;</span></strong>        }        return last;    }}

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