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class Solution {
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 public:
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  ListNode *swapPairs(ListNode *head) {
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    // if head is NULL OR just having a single node, then no need to change
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    // anything
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    if (head == NULL || head->next == NULL) {
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      return head;
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    }
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    ListNode *temp;    // temporary pointer to store head -> next
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    temp = head->next; // give temp what he want
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    head->next = swapPairs(head->next->next); // changing links
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    temp->next = head;                        // put temp -> next to head
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    return temp; // now after changing links, temp act as our head
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  }
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};