[Linked List]Swap Nodes in Pairs

Total Accepted: 73777 Total Submissions: 219963 Difficulty: Medium

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.

 (H) Reverse Nodes in k-Group

/**
 * 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) {
        ListNode* first=head;
        ListNode* second = first ? first->next:NULL;
        ListNode* next=NULL,*pre=NULL;
        while(first && second){
            next = second->next;
            second->next = first;
            first->next = next;
            pre ? pre->next = second: head = second;//有前驱的话，前驱的下一几点指向交换后的两个节点的头，否则，为第一次交换，更新头结点.
            pre  = first;
            first=next;
            second = first ? first->next:NULL;
        }
        return head;
    }
};

Next challenges: (H) Reverse Nodes in k-Group