Leetcode-Swap nodes in pair

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.

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Solution:

The dummy node is used to keep the swap_pair function work in consistent way at the 

very beginning. In other words, to satisfy the function at the very first initialization state.

Noted that we need to pass the pointer by reference so that the structure 

of the list will actually change.

class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        head = dummy;
        while(head) head = swapNodes(head->next);
        head = dummy->next;
        delete dummy;
        return head;
    }
    
    ListNode *swapNodes(ListNode *&head) {
        if(!head || !head->next) return NULL;
        ListNode *tail = head;
        ListNode *nextHead = head->next->next;
        head = head->next;
        head->next = tail;
        tail->next = nextHead;
        return tail;
    }
};