24.Swap Nodes in Pairs (List; Two-Pointers)

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 || !head->next) return head;
        
        ListNode* p1 = head;
        ListNode* p2 = head->next;
        p1->next = p2->next;
        head = p2;
        head->next = p1;
        
        ListNode* current = p1;
        p1 = current->next;
        while(p1 && p1->next){
            p2 = p1->next;
            p1->next = p2->next;
            current->next = p2;
            p2->next = p1;
            
            current = p1;
            p1 = current->next;
        }
        
        return head;
    }
};