Question

import java.util.*;

class Subsets {

 public static List<List<Integer>> findSubsets(int[] nums){
 List<List<Integer>> result = new ArrayList<>();

 Queue<List<Integer>> queue = new LinkedList<>();
 queue.add(new ArrayList<>()); // add empty set to queue
 result.add(new ArrayList<>()); //add empty set to result

 for(int i=0; i<nums.length; i++){

     while(!queue.isEmpty()){
        System.out.println("current result = " + result);

        List<Integer> temp = queue.poll();
        System.out.println("current temp = " + temp);

        System.out.println("before change temp, current result = " + result);

        temp.add(nums[i]);
        System.out.println(i + " add index i value to temp, i= " + temp);
        System.out.println("not yet add to result, current result = " + result);

        result.add(temp);
        System.out.println("after add temp to result, result = " + result);
      }

      //add all elements in result to queue
      int j=0;
      while(j < result.size()){
      queue.add(result.get(j));
      j++;
     }
  }
  return result;
}


 public static void main(String[] args) {
   List<List<Integer>> result = Subsets.findSubsets(new int[] { 1, 3 });
   System.out.println("Here is the list of subsets: " + result);
 }
}

这是代码的输出

current result = [[]]
current temp = []
before change temp, current result = [[]]
0 add index i value to temp, i= [1]
not yet add to result, current result = [[]]
after add temp to result, result = [[], [1]]


current result = [[], [1]]
current temp = []
before change temp, current result = [[], [1]]
1 add index i value to temp, i= [3]
not yet add to result, current result = [[3], [1]]
after add temp to result, result = [[3], [1], [3]]


current result = [[3], [1], [3]]
current temp = [1]
before change temp, current result = [[3], [1], [3]]
1 add index i value to temp, i= [1, 3]
not yet add to result, current result = [[3], [1, 3], [3]]
after add temp to result, result = [[3], [1, 3], [3], [1, 3]]
Here is the list of subsets: [[3], [1, 3], [3], [1, 3]]

我知道这是一种肮脏的代码，但是我不只是想出更好的方法，还需要了解一些我仍然无法弄清楚的部分。

这是获取给定集合子集的代码。例如，当我们得到{1,3} 那么输出应为{}，{1}，{3}，{1,3}

我尝试解决此问题以使用队列，但我的意思是，您可以看到结果输出的第二段

before change temp, current result = [[], [1]]
1 add index i value to temp, i= [3]
not yet add to result, current result = [[3], [1]]

当我查看我的代码时，您绝对可以明白这一点，我对结果什么也不做，只是给temp添加了新值，但是结果突然改变了。

我无法弄清楚我做错了什么，或者我排队时可能有错误的依据？

Answer 1

要解决您所遇到的错误，您必须了解要多次向结果和队列添加相同的列表引用，从而一次又一次地修改这些相同的列表。

将queue.add(result.get(j));更改为queue.add(new ArrayList<>(result.get(j)));，将创建一个新列表，该列表是传递给它的结果列表的副本（而不是像以前一样引用同一列表）。由于现在已复制，因此以后像temp.add(nums[i]);这样的修改将不再修改结果的列表。

import java.util.*;

class Subsets {

 public static List<List<Integer>> findSubsets(int[] nums){
 List<List<Integer>> result = new ArrayList<>();

 Queue<List<Integer>> queue = new LinkedList<>();
 queue.add(new ArrayList<>()); // add empty set to queue
 result.add(new ArrayList<>()); //add empty set to result

 for(int i=0; i<nums.length; i++){

     while(!queue.isEmpty()){
        System.out.println("current result = " + result);

        List<Integer> temp = queue.poll();
        System.out.println("current temp = " + temp);

        System.out.println("before change temp, current result = " + result);

        temp.add(nums[i]);
        System.out.println(i + " add index i value to temp, i= " + temp);
        System.out.println("not yet add to result, current result = " + result);

        result.add(temp);
        System.out.println("after add temp to result, result = " + result);
      }

      //add copy of all elements in result to queue
      int j=0;
      while(j < result.size()){
      queue.add(new ArrayList<>(result.get(j)));
      j++;
     }
  }
  return result;
}


 public static void main(String[] args) {
   List<List<Integer>> result = Subsets.findSubsets(new int[] { 1, 3 });
   System.out.println("Here is the list of subsets: " + result);
 }
}

我无法弄清楚代码，队列和Java中的某些部分

1 个答案: