24. Swap Nodes in Pairs

/**
 * 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 || !head->next) return head;
        ListNode* first = head;
        ListNode* second = head->next;

        first->next = swapPairs(second->next);

        second->next = first;
        return second;
    }
};

• T: $O(N)$
• S: $O(N)$