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# 24. Swap Nodes in Pairs

## [題目連結:] https://leetcode.com/problems/swap-nodes-in-pairs/

## 題目:
Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)


#### [圖片來源:] https://leetcode.com/problems/swap-nodes-in-pairs/
**Example 2:**

Input: head = []
Output: []

**Example 3:**

Input: head = [1]
Output: [1]

## 解題想法:
* 題目要求每兩個node自行交換
* 以example1為例
* 設nextCheck為紀錄下組起始位置
* 目前的兩個node自己處理好交換
* **Step1**: fast.next.next=fast (2指向3改成 2指向1)
* **Step2**: slow.next=fast.next (slow指向2)
* **Step3**: fast.next=nextCheck (最後讓1指向3)
* step2、step3順序不能錯
* 移致下一組
* 示意圖:

#原本: dum、slow         head、fast                    nextCheck
#         0         ->       1         ->    2    ->        3    ->      4
#第一組跑完
#
#      dum、slow                     head、fast       nextCheck
#        0         ->       2     ->    1        ->        3       ->     4
#所以處理下一組為 slow = fast 、fast=nextCheck

## Python:

class ListNode(object):
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
    def insert(self,node):
        if self.next == None:
            self.next=ListNode(node)
        else:
            self.next.insert(node)
    def printList(self):
        head = self
        tmp = []
        while head:
            tmp.append(head.val)
            head = head.next
        print(tmp)
class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        dummy = ListNode(0)
        dummy.next = head
        fast = head
        slow = dummy
        nextCheck = None
        while fast and fast.next:
            #先設值給下個起始點
            nextCheck = fast.next.next
            #斷開自己處理(2指向3改成 2指向1)
            fast.next.next = fast
            slow.next = fast.next 
            fast.next = nextCheck
            #處理下一組
            slow = fast
            fast = nextCheck
        return dummy.next
    
if __name__ == '__main__':
    head = ListNode(1)
    head.insert(2)
    head.insert(3)
    head.insert(4)
    #head.printList()
    result = Solution()
    ans = result.swapPairs(head)
    ans.printList()