Leetcode刷題--Week 5

前言

本週刷題未鏈表排序相關知識點

題目

Given a singly linked list L: L 0→L 1→…→L n-1→L n,
reorder it to: L 0→L n →L 1→L n-1→L 2→L n-2→…
You must do this in-place without altering the nodes’ values.
For example,
Given{1,2,3,4}, reorder it to{1,4,2,3}.

解題思路

1. 首先利用快慢指針找到中間節點
2. 從中間節點截斷鏈表，並將中間節點之後得鏈表進行反轉操作
3. 同時遍歷前半段鏈表和後半段鏈表，將後半段插入前半段

代碼

public void reorderList(ListNode head) {
		if (head == null){
			return;
		}
		ListNode fast = head;
		ListNode slow = head;
		while (fast.next !=null && fast.next.next != null){
			fast = fast.next.next;
			slow = slow.next;
		}
		if (fast == head){
			return;
		}
		ListNode h = new ListNode(0);
		h.next = slow.next;
		slow.next = null;
		ListNode cur = h.next;
		ListNode newHead = new ListNode(0);
		while (cur != null){
			ListNode temp = cur;
			cur = cur.next;
			temp.next = newHead.next;
			newHead.next = temp;
		}
		ListNode preInsert = head;
		cur = newHead.next;
		while (cur != null ){
			ListNode temp = cur;
			cur = cur.next;
			temp.next = preInsert.next;
			preInsert.next = temp;
			preInsert = preInsert.next.next;
		}
	}