24. Swap Nodes in Pairs

24. Swap Nodes in Pairs (Medium)

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

You may not modify the values in the list's nodes. Only nodes itself may be changed.

 

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 the list is in the range [0, 100].
• 0 <= Node.val <= 100

Related Topics:
Linked List

Similar Questions:

• Reverse Nodes in k-Group (Hard)

Solution 1.

// OJ: https://leetcode.com/problems/swap-nodes-in-pairs/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode h, *tail = &h;
        while (head && head->next) {
            auto p = head, q = head->next;
            head = q->next;
            q->next = p;
            tail->next = q;
            tail = p;
        } 
        tail->next = head;
        return h.next;
    }
};