LeetCode24. Swap Nodes in Pairs

題目

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.

思路

設置兩個結點指向p和q，兩個同時移動完成交換結點，問題解決

代碼

/**
 * 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 r = new ListNode(0);
        r.next = head;
        ListNode p = head;
        ListNode q = p.next;
        head = q;
        
        while(p != null && q != null){
            r.next = q;
            p.next = q.next;
            q.next = p;
            
            r = p;
            p = p.next;
            if(p == null || p.next == null)  break;
            q = p.next;
        }
        
        return head;
    }
}