[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.

/**
* 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;
}
};
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