leetcode--24. Swap Nodes in Pairs

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.

题解

交换单链表中相邻的两个节点。

/**
* 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 || !head->next) return head;

ListNode* p = head;
ListNode* q = head->next;

while(q){
swap(p->val, q->val);
if(q->next){
p = q->next;
q = p;
}
q = q->next;
}

return head;
}
};

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
if(head == null || head.next == null) return head;
ListNode p = head.next;
head.next = swapPairs(head.next.next);
p.next = head;
return p;
}
}