24. Swap Nodes in Pairs

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; }}