24. 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;
        }
        else
        {
            ListNode* fast = head->next;
            ListNode* slow = head;

            while(fast!=NULL)
            {
                int temp = slow->val;
                slow->val = fast->val;
                fast->val = temp;

                if(fast->next==NULL)
                {
                    break;
                }
                fast = fast->next->next;
                slow = slow->next->next;
            }

            return head;
        }
    }
};