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 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)
            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;
    }
};