[LeetCode] 24. 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.

class Solution {public:    ListNode* swapPairs(ListNode* head) {        if (head == nullptr || head->next == nullptr) return head;        ListNode virt(-1);        virt.next = head;        ListNode *prev = &virt;        while (head && head->next) {            ListNode *first = head, *second = head->next;            head = second->next;            prev->next = second;            second->next = first;            first->next = head;            prev = first;        }        return virt.next;    }};