#24 Swap Nodes in Pairs

<span style= "font-family:arial, Helvetica, Sans-serif; Background-color:rgb (255, 255, 255); " > Topic link:https://leetcode.com/problems/swap-nodes-in-pairs</span>

Given a linked list, swap every, 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. Modify the values in the list, only nodes itself can be changed.

/** * Definition for singly-linked list. * struct ListNode {*     int val; *     struct ListNode *next; *}; */struct listnode* swappairs (struct listnode* head) {    struct ListNode *prenode, *firstnode, *secondnode;    if (head = = NULL | | head->next = = NULL)        return head;    Firstnode = head;    Secondnode = head->next;    head = Secondnode;    Firstnode->next = secondnode->next;    Head->next = Firstnode;    Prenode = Firstnode;    while (Prenode->next = null && prenode->next->next! = null) {        Firstnode = prenode->next;        Secondnode = firstnode->next;        Prenode->next = Secondnode;        Firstnode->next = secondnode->next;        Secondnode->next = Firstnode;        Prenode = Firstnode;    }    return head;}

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#24 Swap Nodes in Pairs