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.

/** * 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 || head->next == NULL) return head;        ListNode dummy(-1);        dummy.next = head;        for (ListNode *prev = &dummy, *cur = prev->next, *next = cur->next;            next;            prev = cur, cur = cur->next, next = cur ? cur->next : NULL) {            prev->next = next;            cur->next = next->next;            next->next = cur;        }        return dummy.next;    }};