Uebung 3 Loesung

src/main/java/Uebung3_Andres/ExpressionChecker.java

@@ -0,0 +1,44 @@
+package Uebung3_Andres;
+
+import java.util.Scanner;
+
+public class ExpressionChecker {
+
+	private static Stack stack;
+	
+	public static void main(String[] args) {
+		Scanner scanner = new Scanner(System.in);
+		System.out.println("Please enter your expression:");
+		
+		char[] input = scanner.nextLine().toCharArray();
+		stack = new Stack(input.length);
+		scanner.close();
+		boolean isValidExpression = true;
+		for (char letter: input) {
+			if (isOpenBracket(letter)) {
+				stack.push(letter);
+				continue;
+			} 
+			if (isClosingBracket(letter) && !isClosingBracketComplete(letter)) {
+				isValidExpression = false;
+				break;
+			}
+		}
+		//If stack is not empty -> expression was not fully closed = invalid. Ex: ()[
+		isValidExpression = isValidExpression && stack.pop() == '\0';
+		System.out.println(isValidExpression ? "Gültig" : "Klammern ungültig");
+	}
+	
+	private static boolean isOpenBracket(char letter) {
+		return letter == '{' || letter == '[' || letter == '(';
+	}
+	
+	private static boolean isClosingBracket(char letter) {
+		return letter == '}' || letter == ']' || letter == ')';
+	}
+	
+	private static boolean isClosingBracketComplete(char letter) {
+		int diff = (int) letter - (int) stack.pop();
+		return diff == 1 || diff == 2;
+	}
+}

src/main/java/Uebung3_Andres/ExpressionCheckerV2.java

@@ -0,0 +1,46 @@
+package Uebung3_Andres;
+
+import java.util.Scanner;
+
+public class ExpressionCheckerV2 {
+
+	private static Stack stack;
+	
+	public static void main(String[] args) {
+		Scanner scanner = new Scanner(System.in);
+		System.out.println("Please enter your expression:");
+		
+		char[] input = scanner.nextLine().toCharArray();
+		stack = new Stack(input.length);
+		scanner.close();
+		boolean isValidExpression = true;
+		for (char letter: input) {
+			if (isOpenBracket(letter)) {
+				stack.push(letter);
+				continue;
+			} 
+			if (isClosingBracket(letter) && !isClosingBracketComplete(letter)) {
+				isValidExpression = false;
+				break;
+			}
+		}
+		//If stack is not empty -> exp was not closed = invalid
+		isValidExpression = isValidExpression && stack.pop() == '\0';
+		System.out.println(isValidExpression ? "Gültig" : "Klammern ungültig");
+	}
+	
+	private static boolean isOpenBracket(char letter) {
+		return letter == '{' || letter == '[' || letter == '(';
+	}
+	
+	private static boolean isClosingBracket(char letter) {
+		return letter == '}' || letter == ']' || letter == ')';
+	}
+	
+	private static boolean isClosingBracketComplete(char closeBracket) {
+		char openBracket = stack.pop();
+		return (openBracket == '(' && closeBracket == ')') ||
+               (openBracket == '[' && closeBracket == ']') ||
+               (openBracket == '{' && closeBracket == '}');
+	}
+}

src/main/java/Uebung3_Andres/Stack.java

@@ -0,0 +1,21 @@
+package Uebung3_Andres;
+
+public class Stack {
+	
+	private char[] stack;
+	private int pointer;
+	
+	public Stack(int size) {
+		stack = new char[size];
+		pointer = 0;
+	}
+	
+	public void push(char c) {
+		stack[pointer++] = c;
+	}
+	
+	public char pop() {
+		if (pointer == 0) return '\0';
+		return stack[--pointer];
+	}
+}