Swap Nodes in Pairs

思路一：记录遍历列表过程中奇偶性，然后进行交换

/**
 * 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) {
        if(head == nullptr || head->next == nullptr)
            return head;

        ListNode dummy(-1);
        dummy.next = head;
        ListNode *cur = head->next;
        ListNode *curLeft = &dummy;

        int count = 2;
        while(cur)
        {
            if(count % 2 == 0)
            {
                curLeft->next->next = cur->next;
                cur->next = curLeft->next;

                ListNode *tmp = curLeft->next;
                curLeft->next = cur;
                curLeft = tmp;
                cur = cur->next;
            }

            cur = cur->next;
            ++count;
        }

        return dummy.next;
    }
};

思路二：上面思路中使用两个指针，但实际使用三个指针会方便很多

/**
 * 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) {
        if(head == nullptr || head->next == nullptr)
            return head;

        ListNode dummy(-1);
        dummy.next = head;

        for(ListNode *prev = &dummy, *cur=head, *next=head->next; next; prev=cur, cur=cur->next, next=cur ? cur->next:nullptr)
        {
            cur->next = next->next;
            next->next = cur;
            prev->next = next;
        }

        return dummy.next;
    }
};