2.2.5 Remove Duplicates from Sorted List II

Link: https://oj.leetcode.com/problems/remove-duplicates-from-sorted-list-ii/

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

我的思路：思路很簡單。但是代碼還是寫的有問題。

Time: O(n), Space: O(1)

while嵌套一個while。 爲什麼？

我的初始代碼：

public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(head == null) return head;
        ListNode pre = new ListNode(-1);
        pre.next = head;
        ListNode cur = head;
        while(cur.next != null){
            if(cur.next.val != cur.val){
                pre.next = cur;
                pre = cur;
                cur = cur.next;
            }
            else{
                cur = cur.next;
            }
        }
        return head;
    }
}

正確代碼：

public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(head == null) return head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode cur = head;
        while(cur != null){
            //find the place to change pointer
            while(cur.next != null && cur.next.val == cur.val){
                cur = cur.next;
            }
            if(pre.next == cur){
                pre = pre.next;
            }
            else{
                pre.next = cur.next;
            }
            cur = cur.next;
        }
        return dummy.next;
    }
}

相關題目: 注意這一題和Remove Duplicates in Sorted List的區別。

兩道題可以用while嵌套while來做，上一道還可以用while嵌套if/else來做