How can I collect result from nested .forEach in a Java IntStream

djangofan :

I am playing around and trying to solve this problem "Two Sum" with a Java Stream, not using the imperative method:

Given nums = [2, 7, 11, 15], target = 18,
Because nums[1] + nums[2] = 7 + 11 = 18,
return [1, 2].

And here is my partially working code. Can anyone help solve? I just can't figure out how to collect it back up to be returned as a primitive int array :

class Solution { 
    public int[] twoSum(int[] input, int target) {
        IntStream.range(0,  input.length)
           .forEach(i -> {
               IntStream.range(0,  input.length)
                   .filter(j -> i != j && input[i] + input[j] == target)
                   .peek(t -> System.out.println(input[t]))
                   .findFirst();
               }
            );
        return null;
    }
}

And here is the output of the above (coming from peek):

11
7

dehasi :

As I said in comments Streams are not always a perfect solution. But regarding your task, it looks like you need a stream of Pair. Unfortunately Java does not have Tuples (like Scala) that's why I suggest to create a simple Pair class.

class Pair {
    int i, j;
    public Pair(int i, int j) {
        this.i = i;
        this.j = j;
    }
}

Then, work with a stream of pairs

 IntStream.range(0,  input.length)
                .boxed()
                .flatMap(i -> IntStream.range(0,  input.length).boxed()
                                .map(j -> new Pair(i,j))
                )
                        .filter(p -> p.i != p.j)
                        .filter(p -> input[p.i] + input[p.j] == target)
                        .collect(toList());