LeetCode [24] Swap Nodes in Pairs

 24. Swap Nodes in Pairs

Medium

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 1:

Input: head = [1,2,3,4]
Output: [2,1,4,3]

Example 2:

Input: head = []
Output: []

Example 3:

Input: head = [1]
Output: [1]

 

Constraints:

• The number of nodes in the list is in the range [0, 100].
• 0 <= Node.val <= 100

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/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode() {} * ListNode(int val) { this.val = val; } * ListNode(int val, ListNode next) { this.val = val; this.next = next; } * } */ class Solution { public ListNode swapPairs(ListNode head) { if(head==null || head.next==null) return head; ListNode H = new ListNode(); H.next = head; ListNode prev = H, p1 = head, p2 = head.next; while(p2!=null){ ListNode next = p2.next; p2.next = p1; p1.next = next; prev.next = p2; prev = p1; if(next==null || next.next==null) break; p1 = next; p2 = p1.next; } return H.next; } }