[Oj.leetcode] swap-nodes-in-pair

https://oj.leetcode.com/problems/swap-nodes-in-pairs/

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.

/* 단순히 포인터 치환문제. 포인터가 가리키는 노드 자체를 치환하기 위해서는 더블 포인터를 사용하던지, 포인터->next를 변경하여 순서만 바꾸어주면 됨 */ /** * Definition for singly-linked list. * 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; ListNode* first = head; ListNode* second = head->next; ListNode* newHead = second; while(second != NULL) { ListNode* prev = first; first->next = second->next; second->next = first; first = first->next; if(first == NULL) break; second = first->next; if(second != NULL) prev->next = second; } return newHead; } };