title: Remove Duplicates From Sorted List
tags:
- remove-duplicates-from-sorted-list
- No.83
- simple
- list

Description

Given a sorted linked list, delete all duplicates such that each element appear only once.

Example 1:

Input: 1->1->2
Output: 1->2

Example 2:

Input: 1->1->2->3->3
Output: 1->2->3

Corner Cases

• empty list

Solutions

Naive

Here we propose a new method to process list, that is adding a new Head before the original list.

0.
[old head] -> [x] -> [x] -> [x]
1.
[new head] -> [old head] -> [x]

This will make it more convenient for a pointer to traverse and compare.

Since the list is sorted, we only record the maxmium. The running time is O(n).

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        int      m = -2147483648;        
        ListNode a = new ListNode(0);
        a.next     = head;
        ListNode p = a;        
        while (p.next != null) {
            if (m >= p.next.val) {
                p.next = p.next.next;
            }
            else {
                m = p.next.val;
                p = p.next;
            }
        }        
        return a.next;
    }
}