Swap Nodes in Pairs

Description

Given a linked list, swap every two adjacent nodes and return its head.

Example
Given 1->2->3->4, you should return the list as 2->1->4->3.

Challenge
Your algorithm should use only constant space.
You may not modify the values in the list,
only nodes itself can be changed.

Link

Lintcode_ladder

Method

1. x
2. x

Example

1. 1

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    /**
     * @param head a ListNode
     * @return a ListNode
     */
    ListNode* swapPairs(ListNode* head) {
        // Write your code here
        if (!head) {
            return nullptr;
        }
        if (!head->next) {
            return head;
        }
        ListNode dummy(0);
        dummy.next = head;
        ListNode* prev = &dummy;
        ListNode* cur = dummy.next;
        //(dummy)prev-> cur-> curnext -> curnextnext
        while (cur && cur->next) {
            ListNode* rest = cur->next->next;
            ListNode* one = prev->next;
            ListNode* two = cur->next;
            // prev --> two
            // two --> one
            prev->next = two;
            two->next = one;
            // cur
            one->next = rest;
            // advance
            prev = one;
            cur = one->next;
        }
        return dummy.next;
    }
};

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