Is there a way to add a HashMap's value that is a integer array into a ArrayList?

tc100 :

I'm doing this leetcode problem: https://leetcode.com/problems/find-k-pairs-with-smallest-sums/

You are given two integer arrays nums1 and nums2 sorted in ascending order and an integer k.

Define a pair (u,v) which consists of one element from the first array 
and one element from the second array.

Find the k pairs (u1,v1),(u2,v2) ...(uk,vk) with the smallest sums.

Input: nums1 = [1,7,11], nums2 = [2,4,6], k = 3
Output: [[1,2],[1,4],[1,6]] 
Explanation: The first 3 pairs are returned from the sequence: 
             [1,2],[1,4],[1,6],[7,2],[7,4],[11,2],[7,6],[11,4],[11,6]

So my thinking process is to use a HashMap, and a Heap. My HashMap's key is the sum of 2 array's index values. And the value, I will put the 2 values that add up to the sum.

Then I add the sums into the heap, then pop it out K times. That will give me the lowest sums.

Now my problem is that when I try to add the hashmap's value into an ArrayList, it gives me an error. Below is what I have tried.

    public List<List<Integer>> kSmallestPairs(int[] nums1, int[] nums2, int k) {
        List<List<Integer>> ans = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        HashMap<Integer, int[]> map = new HashMap<>();
        PriorityQueue<Integer> minHeap = new PriorityQueue<>();

        for (int i = 0; i < nums1.length; i++) {
            for (int j = 0; j < nums2.length; j++) {
                int sum = nums1[i] + nums2[j];
                map.put(sum, new int[]{nums1[i], nums2[j]});
                minHeap.offer(sum);
            }
        }

        for (int i = 0; i < k; i++) {
            int key = minHeap.poll();
            //list.add(map.get(key)); // ERRORRR
            list.add(map.get(key)[0]);    //gets the first value associated with key
            list.add(map.get(key)[1]);    // gets the second value associated with key
        } 

        ans.add(list);

        return ans;
    }

After researching, the error is because the arraylist has Integer values while my HashMap, I put it as int[]. After searching a bit more, I found

list.add(map.get(key)[0]);
list.add(map.get(key)[1]);

But after doing this, I got the list to show up as: [[1,2,1,4,1,6]].

But I want the answer as: [[1,2],[1,4],[1,6]].

Is there any way to get my list to keep adding a new list? If that makes sense.

Thank you, everyone!

azro :

You may use directly List<Integer> as value of your map to get it easily and put it directly into the result List :

static List<List<Integer>> kSmallestPairs(int[] nums1, int[] nums2, int k) {
    List<List<Integer>> ans = new ArrayList<>();
    HashMap<Integer, List<Integer>> map = new HashMap<>();
    PriorityQueue<Integer> minHeap = new PriorityQueue<>();

    for (int val1 : nums1) {
        for (int val2 : nums2) {
            int sum = val1 + val2;
            map.put(sum, Arrays.asList(val1, val2));
            minHeap.offer(sum);
        }
    }

    for (int i = 0; i < k; i++) {
        int key = minHeap.poll();
        ans.add(map.get(key)); 
    }
    return ans;  //  [[1, 2], [1, 4], [1, 6]]
}

With Java 8 Stream you could even add a shorter way for the loop at the end (replace the for-i loop)

return IntStream.range(0, k).map(e -> minHeap.poll()).mapToObj(map::get)
                                                     .collect(Collectors.toList());

---

And when you get crazy with Stream and don't really care about performances, you could do ... :

import static java.util.Arrays.asList;
import static java.util.Comparator.comparingInt;
import static java.util.stream.Collectors.toList;
import static java.util.stream.IntStream.of;

static List<List<Integer>> kSmallestPairs(int[] nums1, int[] nums2, int k) {
    return of(nums1)
            .mapToObj(v1 -> of(nums2).mapToObj(v2 -> asList(v1, v2)).collect(toList()))
            .flatMap(List::stream)
            .sorted(comparingInt(o -> o.stream().mapToInt(i -> i).sum()))
            .limit(k).collect(toList());
}