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.

題意很簡單，就是遇到第偶數個Node就跟上一個交換

/**
 * 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) {
        
        ListNode* dummy = new ListNode(0);
        ListNode* pre = dummy;
        int count = 1;
        
        dummy->next = head;
                    
        while (head != nullptr)
        {
            //偶數時交換
            if (count % 2 == 0)
            {
                swap(pre->val, head->val);
            }
            
            pre = head;
            head = head->next;
            count++;
        }
        
        head = dummy->next;
        delete dummy;
        return head;
    }
};