24. Swap Nodes in Pairs(Linked List)

Title

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.

Language C

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next; * }; */struct ListNode* swapPairs(struct ListNode* head) {    if(head == NULL || head->next == NULL){        return head;    }    struct ListNode *p, *q, *h, *pre;    // h = (struct ListNode *)malloc(sizeof(struct ListNode));    // h->val =0;    h->next = head;    p = head;    pre = h;    q = p->next;    while(p != NULL && q != NULL){//surprised that must have pre    //This isn't working : p->next = q->next;q->next = p;        pre->next = q;        p->next = q->next;        q->next = p;        pre = pre->next->next;        p = p->next;        q = p->next;    }    return h->next;}

runtime:2ms