Given a linked list, swap every two adjacent nodes and return its head.You may not modify the values in the list’s nodes, only nodes itself may be changed.
Example:
Given 1->2->3->4, you should return the list as 2->1->4->3.
难度：medium
题目：给定链表，交换相邻的两个结点并返回其头结点。不可以修改结点的值，只可以改变结点本身。
思路：加个头结点简化操作。然后就是简单的链表置换。
Runtime: 2 ms, faster than 99.98% of Java online submissions for Swap Nodes in Pairs.Memory Usage: 26 MB, less than 6.66% of Java online submissions for Swap Nodes in Pairs.
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {val = x;}
* }
*/
class Solution {
public ListNode swapPairs(ListNode head) {
ListNode dummyHead = new ListNode(0);
dummyHead.next = head;
ListNode prev = dummyHead;
ListNode next = prev.next;
while (next != null) {
ListNode nextNext = next.next;
if (nextNext != null) {
prev.next = nextNext;
next.next = nextNext.next;
nextNext.next = next;
}
prev = next;
next = prev.next;
}
return dummyHead.next;
}
}