Swap Nodes in Pairs

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

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.

Thoughts

1. 因为head也要翻转，用dummy作为新head

2. 每次把node后面两个翻转，因此牵扯到它后面三个node, 当它后面只有一个时不用翻转，自动退出。

Code

/**
 * 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) {
        ListNode dummy_node(-1);
        auto dummy = &dummy_node, node = dummy;
        dummy->next = head;
        while (node->next != NULL && node->next->next != NULL) {
            auto n1 = node->next, n2 = n1->next, n3 = n2->next;
            node->next = n2;
            n2->next = n1;
            n1->next = n3;
            node = n1;
        }
        return dummy->next;
    }
};

Analysis

做题耗时16min

Errors:

1. node.next.next = tmp

2. 条件是后两个都不为空。

时间复杂度O(n)