19.1.30 [LeetCode 24] Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

题意

将链表中每两个结点两两交换

题解

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode*hh = head,*before=NULL;
        while (head) {
            ListNode*first = head, *second = head->next;
            if (!second)break;
            if (hh == first)hh = second;
            first->next = second->next;
            second->next = first;
            if (before)before->next = second;
            head = first->next;
            before = first;
        }
        return hh;
    }
};