feat coordinate

app/src/main/java/sudoku/Coordinate.java

@@ -0,0 +1,33 @@
+package sudoku;
+
+public class Coordinate {
+
+    private int x;
+    private int y;
+
+    public Coordinate(int row, int col) {
+        this.x = row;
+        this.y = col;
+    }
+
+    public int getX() {
+        return x;
+    }
+
+    public void setX(int x) {
+        this.x = x;
+    }
+
+    public int getY() {
+        return y;
+    }
+
+    public void setY(int y) {
+        this.y = y;
+    }
+
+    public int calculateIndex(int size) {
+        return this.y * size + this.x;
+    }
+
+}