24. Swap Nodes in Pairs

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

# @lc code=start
using LeetCode

function swap_pairs(head::ListNode)::ListNode
    pre = dummpy = ListNode()
    pre.next = head

    while !isnothing(next(pre)) && !isnothing(next(next(pre)))
        a = pre.next
        b = a.next

        # `pre->a->b->b.next` => `pre->b->a->b.next`
        pre.next, b.next, a.next = b, a, b.next

        pre = a
    end

    return dummpy.next
end

# @lc code=end

swap_pairs (generic function with 1 method)

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