1721. 交换链表中的节点

O（n）

注意点k可能大于 n / 2即可

/**
 * 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* p = head;
        int cnt = 1, flag = 0;
        ListNode* q = nullptr, *r = nullptr;
        int n = 0;
        while(p)
        {
            n++;
            p = p->next;
        }
        p = head;
        if(k > n / 2) k = n - k + 1;
        while(p->next)
        {
            if(cnt == k)
            {
                q = p;
                r = head;
                flag = 1;
            }
            if(flag)
                r = r->next, p = p->next;
            else
                p = p->next;
            cnt++;
        }
        if(p == head) return head;
        int tmp = q->val;
        q->val = r->val;
        r->val = tmp;
        return head;
    }
};