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 must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves 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

Companies:
Facebook, Amazon, Microsoft, Bloomberg

Related Topics:
Linked List, Recursion

Similar Questions:

• Reverse Nodes in k-Group (Hard)
• Swapping Nodes in a Linked List (Medium)

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;
    }
};

Or

// 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 dummy, *p = &dummy;
        dummy.next = head;
        while (p->next && p->next->next) {
            auto next = p->next;
            p->next = next->next;
            next->next = p->next->next;
            p->next->next = next;
            p = next;
        }
        return dummy.next;
    }
};