Leetcode Question 108: Swap Nodes in Pairs

Yu's Coding Garden : leetcode Question 108: Swap Nodes in Pairs
Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
http://blog.csdn.net/linhuanmars/article/details/19948569
这道题中用了一个辅助指针作为表头，这是链表中比较常用的小技巧，因为这样可以避免处理head的边界情况，一般来说要求的结果表头会有变化的会经常用这个技巧，大家以后会经常遇到。
public ListNode swapPairs(ListNode head) {

    if(head == null)
        return null;
    ListNode helper = new ListNode(0);
    helper.next = head;
    ListNode pre = helper;
    ListNode cur = head;
    while(cur!=null && cur.next!=null)
    {
        ListNode next = cur.next.next;
        cur.next.next = cur;
        pre.next = cur.next;
        if(next!=null && next.next!=null)
            cur.next = next.next;
        else
            cur.next = next;
        pre = cur;
        cur = next;
    }
    return helper.next;
}

    ListNode *swapPairs(ListNode *head) {

        ListNode *p = new ListNode(0);

        p->next = head;

        head = p;

        while(true){

            if (p->next==NULL){break;}

            if (p->next->next==NULL){break;}

            ListNode* q1 = p->next;

            ListNode* q2 = q1->next;

            q1->next = q2->next;

            q2->next = q1;

            p->next = q2;

            p=q1;

        }

        return head->next;

    }

http://comproguide.blogspot.com/2013/11/pair-swapping-in-linked-list.html
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