24. 两两交换链表中的节点

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        dummy = ListNode(next=head)
        pre, cur = dummy, head
        while cur and cur.next:
            t = cur.next
            cur.next = t.next
            t.next = cur
            pre.next = t
            pre = cur
            cur = pre.next
        return dummy.next

/**
 * 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) {
        ListNode dummy = new ListNode(0, head);
        ListNode pre = dummy, cur = head;
        while (cur != null && cur.next != null) {
            ListNode t = cur.next;
            cur.next = t.next;
            t.next = cur;
            pre.next = t;
            pre = cur;
            cur = pre.next;

        }
        return dummy.next;
    }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode* dummy = new ListNode(0, head);
        ListNode* pre = dummy;
        ListNode* cur = head;
        while (cur != nullptr && cur->next != nullptr) {
            ListNode* t = cur->next;
            cur->next = t->next;
            t->next = cur;
            pre->next = t;
            pre = cur;
            cur = pre->next;
        }
        return dummy->next;
    }
};