leetcode Swap Nodes in Pairs(Java)

题目链接：点击打开链接

类型：链表

解法：递归

/**
* 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 node = head.next;
head.next = swapPairs(head.next.next);
node.next = head;

return node;
}
}

但递归方法空间复杂度不满足常数要求，故考虑解法如下。

/**
* 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) {
ListNode result = new ListNode(0);
result.next = head;
ListNode current = result;

while (current.next != null && current.next.next != null)
{
ListNode first = current.next;
ListNode second = current.next.next;
first.next = second.next;
current.next = second;
second.next = first;
current = current.next.next;
}

return result.next;
}
}