Creating a Linked List

public class ListNode {
    int val;
    ListNode next;

    public ListNode(int val) {
        this.val = val;
        this.next = null;
    }
}

題目：206. Reverse Linked List

連結：https://leetcode.com/problems/reverse-linked-list/description/

• 等級：Easy

解題思路

1. 利用temp進行指向方向轉換

class Solution {
    private ListNode rev(ListNode current,ListNode prev)
    {
        if (current == null) return prev;
        ListNode temp = current.next;
        current.next = prev;
        return rev(temp,current);
    }
    public ListNode reverseList(ListNode head) {
            return rev(head,null);
    }
}

• Time complexity: O(1)
• Space complexity: O(1)

題目：21. Merge Two Sorted Lists

連結：https://leetcode.com/problems/reverse-linked-list/description/

• 等級：Easy

解題思路

1. 利用遞回，並記錄前一個狀態，進行Link merge

class Solution {
    private void mergelist(ListNode curr,ListNode list1,ListNode list2)
    {
        if (list1 == null)
        {
            curr.next = list2;
            return ;
        }
        else if(list2 == null)
        {
            curr.next = list1;
            return;
        }
        else if (list1.val <= list2.val)
        {
            curr.next = list1;
            list1 = list1.next;
        }
        else
        {
            curr.next = list2;
            list2 = list2.next;
        }
        curr = curr.next;
        mergelist(curr,list1,list2);
    }
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode root = new ListNode(0);
        ListNode prev = root;
        mergelist(prev,list1,list2);
        return root.next;
    }
}

• Time complexity: O(1)
• Space complexity: O(1)