<p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 10px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif; font-size: 14.399999618530273px; line-height: 30px;">Given a linked list, swap every two adjacent nodes and return its head.</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 10px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif; font-size: 14.399999618530273px; line-height: 30px;">For example,<br style="box-sizing: border-box;" />Given <code style="box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: 12.800000190734863px; padding: 2px 4px; color: rgb(199, 37, 78); background-color: rgb(249, 242, 244); border-top-left-radius: 4px; border-top-right-radius: 4px; border-bottom-right-radius: 4px; border-bottom-left-radius: 4px;">1->2->3->4</code>, you should return the list as <code style="box-sizing: border-box; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: 12.800000190734863px; padding: 2px 4px; color: rgb(199, 37, 78); background-color: rgb(249, 242, 244); border-top-left-radius: 4px; border-top-right-radius: 4px; border-bottom-right-radius: 4px; border-bottom-left-radius: 4px;">2->1->4->3</code>.</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 10px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif; font-size: 14.399999618530273px; line-height: 30px;">Your algorithm should use only constant space. You may <span style="box-sizing: border-box; font-weight: 700;">not</span> modify the values in the list, only nodes itself can be changed.</p>
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *swapPairs(ListNode *head) {
if(head == NULL)
return head;
ListNode *dummy = new ListNode(-1);
dummy->next = head;
ListNode *p1 = head;
ListNode *p2 = p1->next;
ListNode *prev = dummy;
while(p1 && p2)
{
prev->next = p2;
p1->next = p2->next;
p2->next = p1;
prev = p1;
p1 = p1->next;
if(p1)
p2 = p1->next;
else
break;
}
return dummy->next;
}
};