HashMap :
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for HashMap we have 3 important methods 

1. put
2. get
3. containsKey

put it is used to insert a record
get it is used to fetch the value of the key
containsKey it is used to check wheather a key is there or not

import java.util.*;
class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Try programiz.pro");
        HashMap<Integer,Integer> hm=new HashMap<>();
        hm.put(1,3);
        hm.put(2,6);
        System.out.println(hm.get(2));
        System.out.println(hm.containsKey(2));
        System.out.println(hm.containsKey(10));
    }
}

2 pointers and sliding window
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1. first we need to create one dummy array
2. fill the particular index by using that value

int arr[]={1,2,3,4,6,7,8};
===========================================
import java.util.*;
class HelloWorld {
    public static void main(String[] args) {
       int arr[]={1,5,2,6,7};
       int dummy[]=new int[8];
       
       for(Integer i:arr){
           dummy[i]++;
       }
       
       for(int i=1;i<dummy.length;i++){
           if(dummy[i]==0){
               System.out.println(i);
           }
       }
       //
    }
}
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class Solution {
    public List<Integer> findDuplicates(int[] arr) {
        // code here
        HashMap<Integer,Integer> hm=new HashMap<>();
        for(Integer i:arr){
            if(hm.containsKey(i)){
                int x=hm.get(i);
                x++;
                hm.put(i,x);
            }else{
                hm.put(i,1);
            }
        }
        
        ArrayList<Integer> al=new ArrayList<>();
        
        for(Map.Entry<Integer,Integer> me:hm.entrySet()){
            if(me.getValue()>1){
                al.add(me.getKey());
            }
        }
        return al;
    }
}
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