LeetCode Swap Nodes in Pairs

class Solution {
public:
ListNode *swapPairs(ListNode *head) {
ListNode *a = NULL;
ListNode *b = NULL;
ListNode *tail = NULL;
ListNode *nhead = NULL;
a = head;
while (a != NULL && (b = a->next) != NULL) {
a->next = b->next;
b->next = a;
if (tail != NULL) {
tail->next = b;
} else {
nhead = b;
}
tail = a;
a = a->next;
}
if (nhead == NULL) {
nhead = head;
}
return nhead;
}
};

准备六级，水一发

第二轮：

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.

class Solution {
public:
ListNode *swapPairs(ListNode *head) {
if (head == NULL || head->next == NULL) {
return head;
}
ListNode holder(0);
ListNode* prev = &holder;
ListNode* cur = head;

while (cur != NULL) {
ListNode* next = cur->next;
if (next == NULL) {
break;
}
head = next->next;
next->next = cur;
cur->next = head;
prev->next = next;
prev = cur;
cur = cur->next;
}
return holder.next;
}
};