leetcode 24 swapPairs

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.

/** * 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 dummy(INT_MIN);        ListNode *tail = &dummy;        ListNode *first = head;        while (first != NULL) {            ListNode *second = first->next;            if (second == NULL) break;            ListNode* tmp = second->next;            second->next = first;            tail->next = second;             tail = first;            first = tmp;            } // odd numbers        tail->next = first ? first : NULL;        return dummy.next;    }}; // runtime contribution 25.86%

参考后
I. 指针

/** * 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) {        typedef ListNode* Node;        Node *p = &head;        Node first = NULL;        Node second = NULL;        while ((first = *p) && (second = first->next)) {            first->next = second->next;            second->next = first;            *p = second;            p = &(first->next);        }        return head;    }}; // runtime contribution 25.86%

II. 递归

/** * 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 == NULL) return NULL;        if (head->next == NULL) return head;        ListNode* first = head, *second = head->next;        first->next = second->next;        second->next = first;        first->next = swapPairs(first->next);        return second;    }}; // runtime contribution 25.86%