leetcode-Swap Nodes in Pairs

Difficulty:Medium

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 maynot 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)            return nullptr;        ListNode *front=head;        ListNode *rear=front->next;                while(rear){            swap(front->val,rear->val);            if(!(front=rear->next))                break;            rear=rear->next->next;        }        return head;    }};