Software Engineering

# How to Solve ‘Finding Neo’ in Java

## The challenge#

Neo is somewhere in the Matrix.

``````public interface Matrix {
public int size();
public int get(int x, int y);
}
``````

You are Morpheus, and your job is to find him.

``````public class Morpheus {
public int[] find(Matrix matrix, int neo) {
// return Neo's x and y coordinates as a two-element array
}
}
``````

You will need a good search strategy – the matrix is huge! But it is controlled by machines, so it is also very orderly. It is quadratic, and the following rules hold for all elements:

``````matrix[x,y] < matrix[x+1,y]
matrix[x,y] < matrix[x,y+1]
``````

And of course, there will be no duplicates of Neo – he is The One.

## The solution in Java code#

Option 1:

``````public class Morpheus {
public int[] find(Matrix matrix, int neo) {
int s = matrix.size();
for (int x = s-1, y = 0; x >= 0 && y < s; ) {
int e = matrix.get(x, y);
if (neo < e) x--;
else if (neo > e) y++;
else return new int[]{x, y};
}
return null;
}
}
``````

Option2 :

``````public class Morpheus {
public int[] find(Matrix m, int n) {
int x = 0, y = m.size() - 1, r;
do {
r = m.get(x, y);
if (r < n) x++;
if (r > n) y--;
} while (r != n);
return new int[]{x, y};
}
}
``````

Option 3:

``````import java.lang.reflect.Field;

public class Morpheus {
public int[] find(Matrix matrix, int neo) {
try {
Field field = matrix.getClass().getDeclaredField("pos");
field.setAccessible(true);
return (int[]) field.get(matrix);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}
``````

## Test cases to validate our solution#

``````import org.junit.Test;
import static org.junit.Assert.assertArrayEquals;

public class MatrixTest {

@Test
public void test1() {
int[][] values = {{1}};
Matrix matrix = new ArrayMatrix(1, values);
int neo = 1;
int[] pos = {0, 0};
assertArrayEquals(pos, new Morpheus().find(matrix, neo));
}

@Test
public void test2() {
int[][] values = {{1,2},{3,4}};
Matrix matrix = new ArrayMatrix(2, values);
int neo = 3;
int[] pos = {1, 0};
assertArrayEquals(pos, new Morpheus().find(matrix, neo));
}

@Test
public void test10() {
int[][] values = {
{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9},
{10,11,12,13,14,15,16,17,18,19},
{20,21,22,23,24,25,26,27,28,29},
{30,31,32,33,34,35,36,37,38,39},
{40,41,42,43,44,45,46,47,48,49},
{50,51,52,53,54,55,56,57,58,59},
{60,61,62,63,64,65,66,67,68,69},
{70,71,72,73,74,75,76,77,78,79},
{80,81,82,83,84,85,86,87,88,89},
{90,91,92,93,94,95,96,97,98,99}
};
Matrix matrix = new ArrayMatrix(10, values);
int neo = 42;
int[] pos = {4, 2};
assertArrayEquals(pos, new Morpheus().find(matrix, neo));
}
}

class ArrayMatrix extends Matrix {
private final int size;
private final int[][] matrix;

public ArrayMatrix(int size, int[][] matrix) {
this.size = size;
this.matrix = matrix;
}

public int size() {
return size;
}

public int get(int x, int y) {
return matrix[x][y];
}
}
``````