https://leetcode.cn/problems/swap-nodes-in-pairs/submissions/
/**
 * 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) {
        if(head==nullptr || head->next==nullptr)
        {
            return head;
        }
        ListNode* newHead = head->next;
        head->next=swapPairs(newHead->next);
        newHead->next = head;

        return newHead;
    }
};

https://leetcode.cn/problems/powx-n/submissions/

// 快速幂
class Solution {
public:
    double myPow(double x, int n) {
        if(n==0)
        {
            return 1.0;
        }
        else if(n>0)
        {
            double tmp = myPow(x, n/2);
            return n%2==0 ? tmp*tmp:x*tmp*tmp;
        }
        else
        {
            double tmp = myPow(x, n/2);
            return n%2==0 ? tmp*tmp:(1/x)*tmp*tmp;
        }
    }
};