g0101_0200.s0143_reorder_list.Solution.kt Maven / Gradle / Ivy

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Kotlin-based LeetCode algorithm problem solutions, regularly updated

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package g0101_0200.s0143_reorder_list

// #Medium #Two_Pointers #Stack #Linked_List #Recursion #Data_Structure_II_Day_13_Linked_List
// #Programming_Skills_II_Day_14 #2022_10_09_Time_395_ms_(82.26%)_Space_54_MB_(8.06%)

import com_github_leetcode.ListNode

/*
 * Example:
 * var li = ListNode(5)
 * var v = li.`val`
 * Definition for singly-linked list.
 * class ListNode(var `val`: Int) {
 *     var next: ListNode? = null
 * }
 */
class Solution {
    private var forward: ListNode? = null
    fun reorderList(head: ListNode?) {
        forward = head
        dfs(head)
    }

    private fun dfs(node: ListNode?) {
        if (node != null) {
            dfs(node.next)
            if (forward != null) {
                val next = forward!!.next
                // even case: forward.next coincide with node
                if (next == node) {
                    node.next = null
                } else {
                    node.next = next
                }
                forward!!.next = node
                forward = node.next
            }
            // odd case: forward coincide with node
            if (forward == node) {
                forward!!.next = null
                forward = forward!!.next
            }
        }
    }
}