## Question

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

Result: Accepted Time: 0ms

Here should be some explanations.

struct ListNode* swapPairs(struct ListNode* head) {
struct ListNode * before,* now,* after;
while(after != NULL)
{
before->next = after;
now->next = after->next;
after->next = now;

before = now;
now = now->next;
if(now == NULL)
break;
after = now->next;
}

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$
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