24. Swap Nodes in Pairs

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* swapPairs(ListNode* head) {
ListNode *res = head, *next, *after;
if (head && head->next)
res = head->next;

while (head) {
next = head->next;
if (next) {
after = next->next;
next->next = head;

if (after && after->next)
head->next = after->next;
else
head->next = after;
head = after;
} else {
break;
}
}
return res;
}
};

陷阱多，WA了几次才最终修正。感觉自己总要在debug中纠错，还需要提高姿势水平。