24. Swap Nodes in Pairs

Description

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.

Solution

medium有点扯，注意链表指针的断裂次序

ListNode* swapPairs(ListNode* head) {
    auto dummy = new ListNode(-1), pNode = dummy;
    dummy->next = head;
    while (pNode->next && pNode->next->next) {
        auto pNext = pNode->next;
        pNode->next = pNext->next;
        pNext->next = pNext->next->next;
        pNode->next->next = pNext;
        pNode = pNext;
    }
    return dummy->next;
}