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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(!head) return head;
        ListNode *dummy = new ListNode(0, head);
        ListNode *curr, *next, *prev;
        curr = head; next = curr->next; prev = dummy;
        while(next){
            curr->next = next->next;
            next->next = curr;
            prev->next = next;
            prev = curr;
            curr = curr->next;
            next = (curr) ? curr->next : NULL;
        }
        return dummy->next;
    }
};