将矩阵中的排序合并到 Java 中的列表中

Question

I'm wondering how it's possible to merge sort a matrix into a list?

public class MergeRows {
public static void main(String[] args) {
    int matrix[][] = {
        {1, 3, 5},
        {1, 2, 6},
        {4, 7, 8}
    };
    mergeSort(matrix);
}

public static int mergeSort(int[][] matrix) {
    List<Integer> list = new ArrayList<>();
    for (int i = 0; i < matrix.length; i++) {
        for (int j = 0; j < matrix[i].length; j++) {
            list.addAll(Arrays.asList(matrix));
        }     

        Collections.sort(list);
        System.out.println();
    }
}

I'm quite stuck on this one. It must return [1,1,2,3,4,5,6,7,8].

Answer 1

You must delete the nested loop, matrix[i] is an array so can be passed into Arrays.asList()

public static int mergeSort(int[][] matrix) {
    List<Integer> list = new ArrayList<>();
    for (int i = 0; i < matrix.length; i++) {       
      list.addAll(Arrays.asList(matrix[i]));
    }

    Collections.sort(list);
    System.out.println(Arrays.toString(list.toArray()));
}

Answer 2

The solution in Java is as follows:

List<Integer> list = new List<Integer>();
for (int row = 0; row < matrix.Length; row++)
{
  for (int column = 0; column < matrix[row].Length; column++)
  {
    list.Add(matrix[row][column]);
  }
}

var result = Collections.sort(list);
System.out.println(String.join(",", result));

Two things to note from your solution that are different here. First make sure you do the sort outside the for loops, as you would just be sorting the list each time you add an item and them messing the list up again.

Secondly, you were adding the whole matrix to the list each loop, this method will loop through both dimensions of the matrix and add each element to the new list.

Hope this helps!

Edit Ignore the following it's the C# version, leaving for reference incase. Didn't see the question marked as Java! Solution is for C#.

There is a very quick one line solution to your problem using System.Linq however I shall try to explain it clearly in case you/or anyone viewing doesn't know Linq, forgive me if I'm explaining something you know.

var matrix = new int[][] {
    new int[] { 1, 3, 5 },
    new int[] { 1, 2, 6 },
    new int[] { 4, 7, 8 }
  };

var result = matrix.SelectMany(record => record).OrderBy(record => record).ToList();
Console.WriteLine(String.Join(", ", result));

produces the following in the console: 1, 1, 2, 3, 4, 5, 6, 7, 8

The reason for this is, SelectMany takes a 2d array ([][]) and flattens it into a single array. The record => record portion is simply the selector. In the same way OrderBy(record => record) simply sorts the result of the SelectMany .

SelectMany(record => record) means that you cycle throw each of the arrays and select the array items. For example, if you hand a 2d array as follows:

class Item {
   public string Name { get; set; }
   public string Value { get; set; }
}

var matrix = Item[][] { ... some items ... }

var result = matix.SelectMany(items => items)

Would return a result of Item[] with all the items in a single array from the matrix.

You can take this further to matix.SelectMany(items => items.Select(item => item.Name)) would return a string[] with just the names from each item.

I recommend looking at Linq, it's very useful for operations like this. But beware, you can go wild with it!