力扣143 重排链表 找链表中点 翻转链表 合并链表 线性表法 原地翻转

Problem: 143. 重排链表

🔔 原地翻转：是指在原有的链表上进行操作，而不是不允许建立有序表进行辅助操作。

🍻 线性表辅助法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        ListNode tmp = head;
        // 使用线性表记录链表，可以O(n)的时间进行从前往后和从后往前的遍历
        List<ListNode> list = new ArrayList<>(); 
        while(tmp != null){
            list.add(tmp);
            tmp = tmp.next;
        }
        int i = 0, j = list.size() -1;
        while(i < j){
            list.get(i).next = list.get(j);
            i++;
            if(i == j){
                break;
            }
            list.get(j).next = list.get(i);
            j--; 
        }
        list.get(i).next = null;
    }
}

🍻 找中点 + 翻转链表 + 合并链表

参考图解

class Solution {
    public void reorderList(ListNode head) {
        if (head == null) {
            return;
        }
        ListNode mid = middleNode(head);
        ListNode l1 = head;
        ListNode l2 = mid.next;
        mid.next = null;
        l2 = reverseList(l2);
        mergeList(l1, l2);
    }

    public ListNode middleNode(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    public void mergeList(ListNode l1, ListNode l2) {
        ListNode l1_tmp;
        ListNode l2_tmp;
        while (l1 != null && l2 != null) {
            l1_tmp = l1.next;
            l2_tmp = l2.next;

            l1.next = l2;
            l1 = l1_tmp;

            l2.next = l1;
            l2 = l2_tmp;
        }
    }
}