1. Swap Nodes in Pairs My Submissions QuestionEditorial Solution
   Total Accepted: 95230 Total Submissions: 270562 Difficulty: Easy
   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.

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思路：简单，学会指针间的操作和指向

/** * 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||head->next==NULL)return head;        ListNode *beg = head->next,*p=head,*pre=head;        while(p!=NULL&&p->next!=NULL)        {            ListNode *p_next_next = p->next->next;            pre->next = p->next;            p->next->next = p;            p->next = p_next_next;            pre = p;            p = p->next;        }        return beg;    }};