(24)Swap Nodes in Pairs

class Solution {
public:
    void swap(ListNode* head){
        if(head==nullptr or head->next==nullptr)   return;
        int temp = head->val;
        head->val = head->next->val;
        head->next->val = temp;
        swap(head->next->next);
    }
    ListNode* swapPairs(ListNode* head) {
        swap(head);
        return head;
    }
};