34. Swap Nodes in Pairs

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.

Subscribe to see which companies asked this question

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;
    }
};