A shallow copy deep copy problem of Java power set and list

Seeking power set

Using backtracking, the main view is that each element in the collection is in and out of the list, and a new solution is created in and out;

Import Java.util.arraylist;import Java.util.list;public class P78 {public    list<list<integer>> Subsets (int[] nums) {        list<list<integer>> result=new arraylist<list<integer>> ();        Backtrack (nums,0,new arraylist<integer> (), result);        return result;    }    Backtracking    private void backtrack (int []nums,int start,list<integer> list,list<list<integer>> Result {        //each time the element list is added to result        Result.add (new arraylist<> (list));        for (int i=start;i<nums.length;i++) {     //unordered, each element traverses backwards            List.add (nums[i]);          Joins the current element to the linked list            backtrack (nums,i+1,list,result);            List.remove (List.size ()-1);     Delete the last Element}}}    

shallow copy deep copy problem of list

The List.add (e) method passes in an object that actually holds a reference to the object, so even if the value of E is changed after the Add method executes, the E object that you want to store the different values is going to pass in a deep copy of E at each add (usually using the new E (E)) to achieve

public class Testlistadd {public static void main (String argv[]) {testlistadd temp=new testlistadd ();    Temp.test ();         The public void Test () {/** * Tests string because the string is immutable and therefore teststring= "456"; when it is actually a new object, the "123" that the list first stores is not changed        */String list<string> stringlist=new arraylist<> ();        String teststring= "123";        Stringlist.add (teststring);        Teststring= "456";        System.out.println (stringlist.get (0)); /** * Test object because List holds a object, so after using Add, a will also change the value of a (*/list<student> studentlist=new Arraylis        T<> ();        Student A=new Student (18);        Studentlist.add (a);        A.setage (20);        System.out.println (Studentlist.get (0). Getage ()); /** * Similar to object, to add an immutable List, add only one of its deep copies new ArrayList (a) */list<list<integer>> Lis        Tlist=new arraylist<> ();        List<integer> list1=new arraylist<> (); List1.add (1);       Listlist.add (List1);        Listlist.add (New arraylist<> (List1));        List1.add (2);            for (List l:listlist) {for (Object integer:l) {System.out.print (integer+ "--");        } System.out.println ();        }} class student{private int age;        Student () {} student (int age) {this.age=age;}        public void Setage (int.) {this.age=age;        } public int Getage () {return age; }    }}

A shallow copy deep copy problem of Java power set and list