Leetcode Question 108: 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.

    ListNode *swapPairs(ListNode *head) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        ListNode *p = new ListNode(0);

        p->next = head;

        head = p;

        while(true){

            if (p->next==NULL){break;}

            if (p->next->next==NULL){break;}

            ListNode* q1 = p->next;

            ListNode* q2 = q1->next;

            q1->next = q2->next;

            q2->next = q1;

            p->next = q2;

            p=q1;

        }

        return head->next;

    }

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