24. Swap Nodes in Pairs

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.

自己的方法是将两个reverse，再利用递归，做麻烦了。

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == NULL || head->next == NULL) {
            return head;
        }
        ListNode* next = swapPairs(head->next->next);
        ListNode* prev = NULL;
        ListNode* node = head;
        for (int i = 0; i < 2; i++) {
            ListNode* tmp = node->next;
            node->next = prev;
            prev = node;
            node = tmp;
        }
        head->next = next;
        return prev;
    }
};

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == NULL || head->next == NULL) {
            return head;
        }
        ListNode* next = swapPairs(head->next->next);
        ListNode* node = head->next;
        head->next = next;
        node->next = head;
        return node;
    }
};