Swap-nodes-in-pairs.cpp

/**
 * 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==NULL || head->next==NULL)
            return head;
        ListNode* p=head->next;
        head->next=swapPairs(head->next->next);
        p->next=head;
        return p;
    }
};

// Time Complexity: O(N)
// Space Complexity: O(1)