leetcode 024 —— 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.

思路：三指针 替换 即可

class Solution {public:ListNode* swapPairs(ListNode* head) {if (!head || !head->next) return head;ListNode *newhead = new ListNode(0);ListNode *pre = newhead;pre->next = head;ListNode *l1 = head;ListNode *l2 = head->next;while (l1&&l2){l1->next = l2->next;pre->next = l2;l2->next = l1;pre = l1;l1 = l1->next;if(l1)    l2 = l1->next;}return newhead->next;delete newhead;}};