leetcode刷题笔记一百四十三题 重排链表

leetcode刷题笔记一百四十三题 重排链表

源地址：143. 重排链表

问题描述：

给定一个单链表 L：L0→L1→…→Ln-1→Ln ，
将其重新排列后变为： L0→Ln→L1→Ln-1→L2→Ln-2→…

你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。

示例 1:

给定链表 1->2->3->4, 重新排列为 1->4->2->3.
示例 2:

给定链表 1->2->3->4->5, 重新排列为 1->5->2->4->3.

//整个思路分为三步
//1.使用快慢指针，找到中间结点
//2.后半链表倒叙
//3.合并链表
/**
 * Definition for singly-linked list.
 * class ListNode(_x: Int = 0, _next: ListNode = null) {
 *   var next: ListNode = _next
 *   var x: Int = _x
 * }
 */
object Solution {
    def reorderList(head: ListNode): Unit = {
        if (head != null) {
            var slow = head
        	var fast = head
        
        while (fast != null && fast.next != null){
            fast = fast.next.next
            slow = slow.next    
        }
        
        var current = slow
        var prev: ListNode = null
        
        while (current != null) {
            val next = current.next
            current.next = prev
            prev = current
            current = next
        }
        
        current = head
        while (prev.next != null) {
            val tmp1 = current.next
            current.next = prev
            current = tmp1
            
            val tmp2 = prev.next
            prev.next = current
            prev = tmp2
        }   
        }
    }
}