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Question in Symmetric Tree related to traversal

I am trying to solve a question aka "Symmetric tree"
I have come with the right solution, however I want to know why this code does not work.
I think ArrayList can contain null value and also null value can be compared in method "equals"
Anyone can see where is wrong?(I know this algorithm is not efficient way to do it)
public boolean isSymmetric(TreeNode root) {
if (root == null) {
return true;
}
List<TreeNode> list1 = new ArrayList<>();
List<TreeNode> list2 = new ArrayList<>();
list1 = isMirror1(root, list1);
list2 = isMirror2(root, list2);
for (int i = 0; i < list1.size(); i++) {
if (!list1.get(i).equals(list2.get(i))) {
return false;
}
}
return true;
}
private List<TreeNode> isMirror1 (TreeNode root, List<TreeNode> list) {
if (root == null) {
list.add(null);
return null;
}
list.add(root);
isMirror1(root.left, list);
isMirror1(root.right, list);
return list;
}
private List<TreeNode> isMirror2 (TreeNode root, List<TreeNode> list) {
if (root == null) {
list.add(null);
return null;
}
list.add(root);
isMirror2(root.right, list);
isMirror2(root.left, list);
return list;
}

list1.size() and list2.get(i) can throw NullPointerException
Maybe you can check something like this before loop:
if ( list1==null || list2==null) {
if ( list1==null && list2==null ) return true;
return false;
}
for....
The code is to show the main idea, you can simplify it.
EDIT
first of all, you can implement your own equals method and safely use it:
public boolean equalsWithNulls(TreeNode a, TreeNode b) {
if (a==b) return true;
if ((a==null) || (b==null)) return false;
return a.equals(b);
}

Is there any way to find the duplicate values i.e ArrayList from a hash map?

For eg:-
I have declared a hashMap in the form of:
Map<String, List<Tracks>> dupItems = new LinkedHashMap();
Tracks is a model class that contains name, address, and age.
and I added items in a Tracks
and I added Items as:-
dupItems.add("Project",tracks);
dupItems.add("Report",tracks);
and what I want is a list of duplicate tracks i.e how can I match the items on the basis of values of tracks i.e all values have to be similar. same name, same address, and same age.

If you are looking for the duplicate map values, that is, duplicate lists of Tracks:
One way you can do this is iterate over the values, putting them all in a Set data structure. If you find that the value was already in the set, then it's a duplicate, and you add it to a separate data structure that keeps a record of the duplicate values:
Set<List<Tracks>> findDuplicateValues(Map<String, List<Tracks>> dupItems) {
Set<List<Tracks>> allValues = new HashSet<>();
Set<List<Tracks>> duplicateValues = new HashSet<>();
for (List<Tracks> value : dupItems.values()) {
if (!allValues.add(value)) {
// It's a duplicate!
duplicateValues.add(value);
}
}
return duplicateValues;
}
For this to work reliably, the Tracks class has to implement the equals and hashCode methods. Comparing two objects with the same values using the equals method should return true.
On the other hand, if you are looking for the duplicate Tracks values, no matter which map value list contains them:
you just need to add a loop to the previous method:
Set<List<Tracks>> findDuplicateValues(Map<String, List<Tracks>> dupItems) {
Set<List<Tracks>> allValues = new HashSet<>();
Set<List<Tracks>> duplicateValues = new HashSet<>();
for (List<Tracks> value : dupItems.values()) {
for (Tracks value : values) {
if (!allValues.add(value)) {
// It's a duplicate!
duplicateValues.add(value);
}
}
}
return duplicateValues;
}

I'm assuming we search for duplicate Tracks instances.
public Set<Tracks> findDuplicates(Map<String, List<Tracks>> dupItems) {
Set<Tracks> all = new HashSet<>();
return dupItems.values().stream()
.flatMap(list -> list.stream()) // build a single list of Tracks
.filter(t -> !all.add(t)) // add track to all set but only continue if duplicate
.collect(Collectors.toSet()); // store the remaining tracks in a set (to avoid duplicates in the result
}
A Set will check on duplicates only if the equals() function is correctly implemented.
My eclipse editor generated :
private static class Tracks {
String name;
String address;
int age;
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((address == null) ? 0 : address.hashCode());
result = prime * result + age;
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Tracks other = (Tracks) obj;
if (address == null) {
if (other.address != null)
return false;
} else if (!address.equals(other.address))
return false;
if (age != other.age)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
You can also have a look at Apache Commons EqualsBuilder

How to remove duplicate elements from list<String>? [duplicate]

This question already has answers here:
How do I remove repeated elements from ArrayList?
(40 answers)
Closed 3 years ago.
List list = ["f1,f2","f2,f3","f4,f5","f2,f1","f5,f4"];
output list would be
List uniqueList = ["f1,f2","f2,f3","f4,f5"]

I have another solution . If you dont want to prepare another class to compare values inside List . You can separete each value by comma and sort those data. After that you can again converte them to Set of String
public static void main(String[] args) {
List<String> stringList = Arrays.asList("f1,f2", "f2,f3", "f4,f5", "f2,f1", "f5,f4");
Set<String> result = new HashSet<>();
for (String s : stringList) {
String[] elements = s.split(",");
Arrays.sort(elements);
result.add(Arrays.toString(elements));
}
for (String e : result){
System.out.println(e);
}
}

Using an additional class:
static class Pair {
String a, b;
Pair(String s) {
String[] arr = s.split(",");
this.a = arr[0];
this.b = arr[1];
}
static String pairToString(Pair p) {
return p.a + "," + p.b;
}
#Override
public int hashCode() {
return Objects.hash(a, b) + Objects.hash(b, a);
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Pair p = (Pair) o;
return (p.a.equals(a) && p.b.equals(b)) || (p.b.equals(a) && p.a.equals(b));
}
}
Now you can use:
public static void main(String[] args) {
List<String> list = Arrays.asList("f1,f2", "f2,f3", "f4,f5", "f2,f1", "f5,f4");
List<String> strings = list
.stream()
.map(Pair::new)
.distinct()
.map(Pair::pairToString)
.collect(Collectors.toList());
}

I have created a class to model the pairs and override the equals method to treat "f1,f2" and "f2,f1" as equals and then found out the duplicates using HashSet.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
public class so1 {
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[] {"f1,f2","f2,f3","f4,f5","f2,f1","f5,f4"});
List<pair> plist = new ArrayList<pair>();
for (int i = 0; i < list.size(); i++) {
plist.add(new pair(list.get(i)));
}
HashSet<pair> hs = new HashSet<pair>();
for (int i = 0; i < plist.size(); i++) {
if(!hs.add(plist.get(i))){
System.out.println("Found duplicate "+plist.get(i).toString());
}
}
List<String> uniqueItems = new ArrayList<String>();
for (Iterator iterator = hs.iterator(); iterator.hasNext();) {
pair pair = (pair) iterator.next();
uniqueItems.add(pair.toString());
}
System.out.println(uniqueItems);
}
}
class pair{
pair(String inp){
String[] tokens = inp.split(",");
Arrays.sort(tokens);
for(String t: tokens){
elements.add(t);
}
}
List<String> elements = new ArrayList<>();
#Override
public String toString() {
return ""+elements;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((elements == null) ? 0 : elements.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
pair other = (pair) obj;
if (elements == null) {
if (other.elements != null)
return false;
} else if (!elements.equals(other.elements))
return false;
return true;
}
}

Here are a couple answers from https://www.geeksforgeeks.org/how-to-remove-duplicates-from-arraylist-in-java/
Java 7
// Create a new ArrayList
ArrayList<String> uniqueList = new ArrayList<String>();
// Traverse through the first list
for ( element : list) {
// If this element is not present in uniqueList
// then add it
if (!uniqueList.contains(element)) {
uniqueList.add(element);
}
}
Java 8
List<String> uniqueList = list.stream()
.distinct()
.collect(Collectors.toList());

How to get the index of a specific item in an ArrayList?

I fill up an Array List with some numbers and want to find a specific number that is in the Array List and get its position (the index) in my Array List.
Any example would be great!
for example
ProClon.indexOf(spro.getId(id));

First override equals() method with the specified field. then You can use indexOf.(object)
class A {
int i;
// other fields
public A(int i) {
this.i = i;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
A a = (A) o;
return i == a.i;
}
#Override
public int hashCode() {
return i;
}
}
List<A> list = new ArrayList<>();
list.indexOf(new A(3));

check the api of arrayList. indexOf(Object o); does exactly what you need.
http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#indexOf(java.lang.Object)

Just use method indexOf in array
arrayName.indexOf(object)

Java Compare Two List's object values?

I have two list **ListA<MyData> listA = new ArrayList<MyData>()** and ListB<MyData> listB = new ArrayList<MyData>() both contain object of type MyData and MyData contain these variables.
MyData {
String name;
boolean check;
}
ListA and ListB both contains MyData objects ,now I have to compare both the list's object values here name as well check variable like if ListA contains these object values
ListA = ["Ram",true],["Hariom",true],["Shiv",true];
and ListB also contain
ListB = ["Ram",true],["Hariom",true],["Shiv",true];
then i have to compare lists and return false because both list are same
But if ListA contains
ListA = ["Ram",true],["Hariom",true],["Shiv",false];
and ListB Contain
ListB = ["Ram",true],["Hariom",true],["Shiv",true];
then I have to compare lists and return true because both list are not same
or vice-versa so any slight change in the any list values I have to return true.
One thing I have to mentioned here objects can be in any order.

It's not the most efficient solution but the most terse code would be:
boolean equalLists = listA.size() == listB.size() && listA.containsAll(listB);
Update:
#WesleyPorter is right. The solution above will not work if duplicate objects are in the collection.
For a complete solution you need to iterate over a collection so duplicate objects are handled correctly.
private static boolean cmp( List<?> l1, List<?> l2 ) {
// make a copy of the list so the original list is not changed, and remove() is supported
ArrayList<?> cp = new ArrayList<>( l1 );
for ( Object o : l2 ) {
if ( !cp.remove( o ) ) {
return false;
}
}
return cp.isEmpty();
}
Update 28-Oct-2014:
#RoeeGavriel is right. The return statement needs to be conditional. The code above is updated.

ArrayList already have support for this, with the equals method. Quoting the docs
...
In other words, two lists are defined to be equal if they contain the same elements in the same order.
It does require you to properly implement equals in your MyData class.
Edit
You have updated the question stating that the lists could have different orders. In that case, sort your list first, and then apply equals.

I got this solution for above problem
public boolean compareLists(List<MyData> prevList, List<MyData> modelList) {
if (prevList.size() == modelList.size()) {
for (MyData modelListdata : modelList) {
for (MyData prevListdata : prevList) {
if (prevListdata.getName().equals(modelListdata.getName())
&& prevListdata.isCheck() != modelListdata.isCheck()) {
return true;
}
}
}
}
else{
return true;
}
return false;
}
EDITED:-
How can we cover this...
Imagine if you had two arrays "A",true "B",true "C",true and "A",true "B",true "D",true. Even though array one has C and array two has D there's no check that will catch that(Mentioned by #Patashu)..SO for that i have made below changes.
public boolean compareLists(List<MyData> prevList, List<MyData> modelList) {
if (prevList!= null && modelList!=null && prevList.size() == modelList.size()) {
boolean indicator = false;
for (MyData modelListdata : modelList) {
for (MyData prevListdata : prevList) {
if (prevListdata.getName().equals(modelListdata.getName())
&& prevListdata.isCheck() != modelListdata.isCheck()) {
return true;
}
if (modelListdata.getName().equals(prevListdata.getName())) {
indicator = false;
break;
} else
indicator = true;
}
}
}
if (indicator)
return true;
}
}
else{
return true;
}
return false;
}

First, implement the MyData.equals(Object o) and MyData.hashCode() methods.
Once you implemented the equals method, you can iterate over the lists as follows:
if(ListA == null && ListB == null)
return false;
if(ListA == null && ListB != null)
return true;
if(ListA != null && ListB == null)
return true;
int max = ListA.size() > ListB.size() ? ListA.size() : ListB.size();
for(int i = 0; i < max; i++) {
myData1 = ListA.get(i);
myData2 = ListB.get(i);
if(!myData1.equals(myData2)) {
return true;
}
}
return false;

I found a very basic example of List comparison at List Compare
This example verifies the size first and then checks the availability of the particular element of one list in another.

This can be done easily through Java8 using forEach and removeIf method.
Take two lists. Iterate from listA and compare elements inside listB
Write any condition inside removeIf method.
Hope this will help
listToCompareFrom.forEach(entity -> listToRemoveFrom.removeIf(x -> x.contains(entity)));

Override the equals method in your class and use Collection#equals() method to check for equality.

See if this works.
import java.util.ArrayList;
import java.util.List;
public class ArrayListComparison {
public static void main(String[] args) {
List<MyData> list1 = new ArrayList<MyData>();
list1.add(new MyData("Ram", true));
list1.add(new MyData("Hariom", true));
list1.add(new MyData("Shiv", true));
// list1.add(new MyData("Shiv", false));
List<MyData> list2 = new ArrayList<MyData>();
list2.add(new MyData("Ram", true));
list2.add(new MyData("Hariom", true));
list2.add(new MyData("Shiv", true));
System.out.println("Lists are equal:" + listEquals(list1, list2));
}
private static boolean listEquals(List<MyData> list1, List<MyData> list2) {
if(list1.size() != list2.size())
return true;
for (MyData myData : list1) {
if(!list2.contains(myData))
return true;
}
return false;
}
}
class MyData{
String name;
boolean check;
public MyData(String name, boolean check) {
super();
this.name = name;
this.check = check;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + (check ? 1231 : 1237);
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
MyData other = (MyData) obj;
if (check != other.check)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}

You can subtract one list from the other using CollectionUtils.subtract, if the result is an empty collection, it means both lists are the same. Another approach is using CollectionUtils.isSubCollection or CollectionUtils.isProperSubCollection.
For any case you should implement equals and hashCode methods for your object.

Using java 8 removeIf to compare similar items
public int getSimilarItems(){
List<String> one = Arrays.asList("milan", "dingo", "elpha", "hafil", "meat", "iga", "neeta.peeta");
List<String> two = new ArrayList<>(Arrays.asList("hafil", "iga", "binga", "mike", "dingo")); //Cannot remove directly from array backed collection
int initial = two.size();
two.removeIf(one::contains);
return initial - two.size();
}

Logic should be something like:
First step: For class MyData implements Comparable interface, override the compareTo method as per the per object requirement.
Second step: When it comes to list comparison (after checking for nulls),
2.1 Check the size of both lists, if equal returns true else return false, continue to object iteration
2.2 If step 2.1 returns true, iterate over elements from both lists and invoke something like,
listA.get(i).compareTo(listB.get(i))
This will be as per the code mentioned in step-1.

It's been about 5 years since then and luckily we have Kotlin now.
Comparing of two lists now looks is as simple as:
fun areListsEqual(list1 : List<Any>, list2 : List<Any>) : Boolean {
return list1 == list2
}
Or just feel free to omit it at all and use equality operator.

I know it's old question but in case anyone needs it. I use this in my application and it works well. i used it to check if the cart has been changed or not.
private boolean validateOrderProducts(Cart cart) {
boolean doesProductsChanged = false;
if (originalProductsList.size() == cart.getCartItemsList().size()) {
for (Product originalProduct : originalProductsList) {
if (!doesProductsChanged) {
for (Product cartProduct : cart.getCartProducts()) {
if (originalProduct.getId() == cartProduct.getId()) {
if (originalProduct.getPivot().getProductCount() != cartProduct.getCount()) {
doesProductsChanged = true;
// cart has been changed -> break from inner loop
break;
}
} else {
doesProductsChanged = false;
}
}
} else {
// cart is already changed -> break from first loop
break;
}
}
} else {
// some products has been added or removed (simplest case of Change)
return true;
}
return doesProductsChanged;
}

String myData1 = list1.toString();
String myData2 = list2.toString()
return myData1.equals(myData2);
where :
list1 - List<MyData>
list2 - List<MyData>
Comparing the String worked for me. Also NOTE I had overridden toString() method in MyData class.

I think you can sort both lists and convert to List if some of them was a HashSet colleciton.
java.utils.Collections package lets you do it.
List<Category> categoriesList = new ArrayList<>();
Set<Category> setList = new HashSet<>();
Collections.sort(categoriesList);
List<Category> fileCategories = new ArrayList<>(setList);
Collections.sort(fileCategories);
if(categoriesList.size() == fileCategories.size() && categoriesList.containsAll(fileCategories)) {
//Do something
}