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.

Code:

ListNode* swapPairs(ListNode* head) {
if (!head)
return NULL;
ListNode *preNode = new ListNode(0);
preNode ->next = head;

ListNode *m_preNode = preNode;
ListNode *p = head;
ListNode *q = p->next;

while (p&&q)
{
preNode->next = q;
p->next = q->next;
q->next = p;

preNode = p;
p = preNode->next;
//要注意检测p是否为空值，否则对q赋值就会出现错误
if (p)
q = p->next;
else
q = NULL;
}
return m_preNode->next;
}