78 Subsets

Given a set of distinct integers, nums, return all possible subsets.

Note: Elements in a subset must be in non-descending order. The solution set must not contain duplicate subsets. For example, If nums = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

Analysis

Bug

1.对接口的实例化：

//实例最外层即可
List<List<Integer>> res = new ArrayList<List<Integer>>();
Queue<List<Integer>> q = new LinkedList<List<Integer>>();
//未实例化的对象用接口型的Reference
List<Integer> al = q.poll();

Code

public List<List<Integer>> subsets(int[] nums) {

        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Queue<List<Integer>> q = new LinkedList<List<Integer>>();
        //create a hash map to store nums' index info
        Map<Integer, Integer> indexOfnums = new HashMap<Integer, Integer>();

        //initialize
        q.add(new ArrayList<Integer>());
        Arrays.sort(nums);

        //store index of every int in nums[]
        for(int i=0; i<nums.length; i++) {
            indexOfnums.put(nums[i], i);
        }

        //funny part
        while(!q.isEmpty()) {
            int sizeOfcurrentLevel = q.size();
            for(int i=0; i < sizeOfcurrentLevel; i++) {
                List<Integer> al = q.poll();
                res.add(al);

                //find the index of int who need to be appended
                int index = 0;
                if(al.size() != 0){
                    Integer last = al.get( al.size() - 1 );
                    index = indexOfnums.get(last) + 1;
                }
                else {
                    index = 0;
                }

                //if nums[index] is a valid index, append all int to al
                if(index < nums.length) {
                    for(int j= index; j < nums.length; j++) {
                        ArrayList<Integer> temp = new ArrayList<Integer>(al);
                        temp.add(nums[j]);
                        q.add(temp);  
                    }
                }
            }
        }
        return res;
    }