Noch eine Version für Aufgabe 3

src/main/java/Uebung_03_Zehra/BracketChecker.java

@@ -0,0 +1,52 @@
+package Uebung_03_Zehra;
+
+public class BracketChecker {
+    Stack stack;
+
+    public BracketChecker(Stack stack, String expression) {
+        this.stack = stack;
+        check(expression);
+    }
+
+    /*
+Check whether the bracket is an opening one. if so: push onto the stack;
+ if not: see whether the last bracket on the stack, which is an opening one fits with the one currently under observation from
+ the expression
+ */
+    public boolean check(String expression) {
+        for (int i = 0; i < expression.length(); i++) {
+            if (expression.charAt(i) == '(' || expression.charAt(i) == '{' || expression.charAt(i) == '[') {
+                this.stack.push(expression.charAt(i));
+                System.out.println(expression.charAt(i) + "auf den stack gelegt");
+            } else if (expression.charAt(i) == ')' || expression.charAt(i) == '}' || expression.charAt(i) == ']') {
+                char openingBracket = this.stack.pop();
+
+                //create a bracket pair from the opening bracket from the stack, and the closing bracket currently being observed from the expression
+                String brackets = "" + openingBracket + expression.charAt(i);
+                System.out.println("Passen die zusammen? " + brackets);
+
+                //check whether the bracket pair is harmonious
+                if (!(brackets.equals("()") || brackets.equals("{}") || brackets.equals("[]"))) {
+                    System.out.println("nein tun sie nicht - Klammern ungültig");
+                    return false;
+                } else {
+                    System.out.println("ja sieht gut aus");
+                }
+            }
+        }
+        /*if there are still unpopped chars on the stack, they must be opening brackets,
+        that have no corresponding closing brackets in the expression. Hence the expression is not valid.
+        */
+        System.out.println("Der Stackpointer zeigt aktuell auf den index" + stack.stackPointerTop);
+        if (stack.stackPointerTop > 0) {
+            System.out.println("Es gibt öffnende Klammern, zu denen es keine schließenden gibt. Ausdruck ungültig");
+            return false;
+        }
+
+        /* if we got here that means that no more opening brackets are on the stack to check for a corresponding closing bracket in the expression.
+        hence the expression is valid
+                */
+        System.out.println("Ausdruck ist gültig");
+        return true;
+    }
+}

src/main/java/Uebung_03_Zehra/Main.java

@@ -0,0 +1,19 @@
+package Uebung_03_Zehra;
+
+import java.util.Scanner;
+
+public class Main {
+
+    public static void main(String[] args) {
+      Stack stack = new Stack();
+        Scanner scanner = new Scanner(System.in);
+        String expression = scanner.nextLine();
+      BracketChecker bracketChecker = new BracketChecker(stack, expression);
+      System.out.println("Bitte geben Sie ihren Ausdruck ein");
+
+
+
+
+
+    }
+}

src/main/java/Uebung_03_Zehra/Stack.java

@@ -0,0 +1,27 @@
+package Uebung_03_Zehra;
+
+import java.util.Scanner;
+
+public class Stack{
+
+    public char[] stack;
+    public int stackPointerTop=0;
+
+    public Stack() {
+        this.stack = new char[100];
+        this.stackPointerTop=0;
+    }
+
+    public void push(char c){
+        stack[stackPointerTop]=c;
+        stackPointerTop++;
+
+    }
+    public char pop(){
+        if(stackPointerTop>=1){
+            return stack[--stackPointerTop];
+        }
+      return '\0'; // '\0' stands for an empty character (null character)
+
+    }
+}