add diagonal constraint

app/src/main/java/sudoku/constraint/DiagonalConstraint.java

@@ -0,0 +1,24 @@
+package sudoku.constraint;
+
+import sudoku.structure.Sudoku;
+
+public class DiagonalConstraint implements IConstraint {
+
+	@Override
+	public boolean canBePlaced(Sudoku s, int x, int y, int newSymbolIndex) {
+		if (x == y) {
+			for (int i = 0; i < s.getSize(); i++) {
+				if (s.getCell(i, i).getSymbolIndex() == newSymbolIndex)
+				return false;
+			}
+		} else if (s.getSize() - x == y) {
+			for (int i = 0; i < s.getSize(); i++) {
+				if (s.getCell(s.getSize() - i, i).getSymbolIndex() == newSymbolIndex)
+				return false;
+			}
+		}
+		// not in diagonal
+		return true;
+	}
+
+}