*Java* straightforward iterative solution (4ms)

  • 0

    Key ideas:

    • sort the array
    • store obtained list up to index i (exclusive)
    • for index i, skip all values equal to i and count number of skipped items. Consider all items with same value one block and process all of them at once
    • It's now a simple math combination problem: from 0 to i-1, we have a list of results res, at i (after skipping duplicates), we have another list of results innerList, just add all combinations of res and innerList

    For example,

    e.g., nums = [1, 2, 3, 3, ...]
    before i=2, we have res = [[ ], [1], [2], [1,2]]
    when i=2, skip value 3 and move i to index 3, skipped indices=1
    innerList = [[ ], [3], [3,3]]
    add all combinations of `res` and `innerList` and the outcome becomes the new res

    Code in Java:

    public List<List<Integer>> subsetsWithDup(int[] nums) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> innerList = new ArrayList<Integer>();
        for(int i=0, L=nums.length; i<L; i++) {
        	int skipped = 0; // number of duplicate values
    		while(i+1<L && nums[i]==nums[i+1]) { // skip duplicates
    		List<List<Integer>> newList = new ArrayList<List<Integer>>(); // to store new result including i;
    		for(List<Integer> left : res) {
    			innerList = new ArrayList<Integer>();
    			for(int j=0; j<=skipped; j++) { // add all duplicate values
    				List<Integer> combined = new ArrayList<Integer>(left);
    		res = newList; // update res
        return res;

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