package katas.kotlin.leetcode.swap_nodes_in_pairs

import katas.kotlin.leetcode.ListNode
import katas.kotlin.leetcode.listNodes
import datsok.shouldEqual
import org.junit.Test

/**
 * https://leetcode.com/problems/swap-nodes-in-pairs
 */
class SwapNodesInPairsTests {
    @Test fun `swap every two adjacent nodes`() {
        listNodes(1, 2).swapPairs() shouldEqual listNodes(2, 1)
        listNodes(1, 2, 3, 4).swapPairs() shouldEqual listNodes(2, 1, 4, 3)
        listNodes(1, 2, 3, 4, 5, 6).swapPairs() shouldEqual listNodes(2, 1, 4, 3, 6, 5)
        listNodes(1, 2, 3, 4, 5, 6, 7, 8).swapPairs() shouldEqual listNodes(2, 1, 4, 3, 6, 5, 8, 7)

        listNodes(1).swapPairs() shouldEqual listNodes(1)
        listNodes(1, 2, 3).swapPairs() shouldEqual listNodes(2, 1, 3)
        listNodes(1, 2, 3, 4, 5).swapPairs() shouldEqual listNodes(2, 1, 4, 3, 5)
    }
}

private fun ListNode.swapPairs(): ListNode {
    var first: ListNode? = this
    var second = first?.next
    var third = second?.next
    var fourth = third?.next
    val head = second ?: return first!!

    second.next = first
    first?.next = fourth ?: third

    do {
        first = third
        second = first?.next
        third  = second?.next
        fourth = third?.next

        second?.next = first
        first?.next = fourth ?: third
    } while (third != null)

    return head
}
