题目

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.

分析

将链表中的相邻两个元素交换，用好指针即可完成，需要多次尝试，避免出错

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* swapPairs(struct ListNode* head) {
    struct ListNode * p=head,*q=head;
    if(p==head&&p!=NULL&&p->next!=NULL)
    {
        head=p->next;
        p->next=head->next;
        head->next=p;
        p=p->next;
    }
    while(p!=NULL && p->next!=NULL)
    {
        printf("%d\n",p->val);
        q->next=p->next;
        p->next=p->next->next;
        q->next->next=p;
        q=p;
        p=p->next;
    }
    return head;
}