Remove Duplicates from Sorted List

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

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

Solution:

稍微修改上一个代码即可，遇到次数多于1的，先输出一个，然后将hashmap中标记一下（这里我就是将其value改为0），后面就不输出它即可。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
          HashMap<Integer,Integer> map = new HashMap<Integer,Integer>();
        ListNode cur = head;
        while(cur != null){
            if(map.containsKey(cur.val)){
                map.put(cur.val,map.get(cur.val) + 1);
            }else{
                map.put(cur.val,1);
            }
            cur = cur.next;
        }
        ListNode header = new ListNode(-1);
        header.next = head;
        cur = header;
        while(cur.next != null){
            if(map.get(cur.next.val) > 1){
                map.put(cur.next.val,0);
                cur = cur.next;
            }
            else if(map.get(cur.next.val) == 1){
                cur = cur.next;
            }else{
                cur.next = cur.next.next;
            }
        }
        return header.next;
    }
}

 我发现可以简化。。。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
        HashMap<Integer,Integer> map = new HashMap<Integer,Integer>();
        ListNode header = new ListNode(-1);
        header.next = head;
        ListNode cur = header;
        while(cur.next != null){
            if(map.containsKey(cur.next.val)){
                cur.next = cur.next.next;
            }else{
                map.put(cur.next.val,1);
                cur = cur.next;
            }
        }
        return header.next;
    }
}

 第二遍： 由于linkedlist 已经被sorted，所以可以直接用双指针来完成。使用constant extra space。

public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int value = Integer.MAX_VALUE;
        ListNode cur = head, per = head;
        while(per != null){
            if(per.val != value){
                value = per.val;
                cur = per;
                per = per.next;
            }else{
                per = per.next;
                cur.next = per;
            }
        }
        return head;
    }
}