Given a linked list, swap every two adjacent nodes and return its head.
For example, Given 1->2->3->4, you should return the list as 2->1->4->3.
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ struct ListNode { int val; ListNode *next; ListNode(int x) :val(x), next(NULL) {} }; class Solution { public: ListNode *swapPairs(ListNode *head) { if(head == NULL || head->next == NULL) { return head; } else { ListNode *p = head->next; ListNode *q = head->next->next; p->next = head; head->next = swapPairs(q); return p; } } };
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