LeetCode 1721. 交换链表中的节点（快慢指针）

示例 1：
输入：head = [1,2,3,4,5], k = 2
输出：[1,4,3,2,5]

示例 2：
输入：head = [7,9,6,6,7,8,3,0,9,5], k = 5
输出：[7,9,6,6,8,7,3,0,9,5]

示例 3：
输入：head = [1], k = 1
输出：[1]

示例 4：
输入：head = [1,2], k = 1
输出：[2,1]

示例 5：
输入：head = [1,2,3], k = 2
输出：[1,2,3]
 
提示：
链表中节点的数目是 n
1 <= k <= n <= 10^5
0 <= Node.val <= 100

/**
 * 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* swapNodes(ListNode* head, int k) {
        ListNode* cur = head, *prev = head;
        ListNode* a, *b;
        int n = k-1;
        while(n--)
        {
            cur = cur->next;
        }
        a = cur;
        while(cur->next != NULL)
        {
            cur = cur->next;
            prev = prev->next;
        }
        b = prev;
        swap(a->val, b->val);
        return head;
    }
};