24.Swap Nodes in Pairs

Medium

3591208Add to ListShare

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

Example 1:

Input: head = [1,2,3,4]
Output: [2,1,4,3]

Example 2:

Input: head = []
Output: []

Example 3:

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

Constraints:

• The number of nodes in thelistis in the range[0, 100]
  .
• 0 <= Node.val <= 100

Follow up:Can you solve the problem withoutmodifying the values in the list's nodes? (i.e., Only nodes themselves may be changed.)

連結串列的特點:是把指標指向轉換

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        if(head==NULL || head->next==NULL){
            return head;
        }
        ListNode *current = dummy;
        while(current->next!=NULL && current->next->next!=NULL){
            ListNode *first = current->next;
            ListNode *second = current->next->next;
            
            first->next = second->next;
            second->next = first;
            current->next = second;
            
            current = current->next->next;
        }
    return dummy->next;    
    }
};