feat: menambahkan contoh dari infix ke postfix

src/main/java/algorithm/datastructure/Stack/BalancedBracket.java

@@ -0,0 +1,61 @@
+package algorithm.datastructure.Stack;
+
+/**
+ * tanda kurung bersarang adalah problem dimana menentukan
+ * apakah urutan tanda kurung bersarang dengan benar
+ * urutan tanda kurung s dianggap benar bersarang
+ * jika salah satu kondisi berikut
+ * - S kosong
+ * - s memiliki bentuk
+ * (U) atau [U] atau {U}
+ * dimana U ini adalah string bersarang yang benar
+ */
+import java.util.Stack;
+
+class BalancedBracket {
+  public static boolean telahTersambung(char bracketKiri, char bracketKanan) {
+    char[][] bracketSambungan = {
+      {'(', ')'},
+      {'[', ']'},
+      {'{', '}'},
+      {'<', '>'},
+    };
+    for (char[] bracketTersambung : bracketSambungan) {
+      if (bracketTersambung[0] == bracketKiri && bracketTersambung[1] == bracketKanan) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  public static boolean bracketSesuai(String brackets) {
+    if (brackets == null) {
+      throw new IllegalArgumentException("bracket null");
+    }
+    Stack<Character> bracketStack = new Stack<>();
+    for (char bracket : brackets.toCharArray()) {
+      switch (bracket) {
+        case '(':
+        case '{':
+        case '[':
+          bracketStack.push(bracket);
+          break;
+        case ')':
+        case '}':
+        case ']':
+          if (bracketStack.isEmpty() || !telahTersambung(bracketStack.pop(), bracket)) {
+            return false;
+          }
+          break;
+        default:
+          return false;
+      }
+    }
+    return bracketStack.isEmpty();
+  }
+
+  public static void main(String[] arags) {
+    assert bracketSesuai("[()]{}{[()()]()}");
+    assert !bracketSesuai("[(])");
+  }
+}

src/main/java/algorithm/datastructure/Stack/InfixToPostfix.java

@@ -0,0 +1,61 @@
+package algorithm.datastructure.Stack;
+
+/**
+ * notasi infix adalah notasi dimana operator berada diantara
+ * operan yang diperlukan. notasi postfix adalah jenis notasi
+ * dimana operator muncul setelah operan
+ * 
+ * ekspresi infix -> A + B * C + D
+ * ke eksprei postfix menjadi -> ABC * + D +
+ */
+import java.util.Stack;
+
+public class InfixToPostfix {
+
+  public static String infixToPostfix(String ekspresiInfix) throws  Exception {
+    if (!BalancedBracket.bracketSesuai(ekspresiInfix)) {
+      throw new Exception("ekspresi salah");
+    }
+    StringBuilder output = new StringBuilder();
+    Stack<Character> stack = new Stack<>();
+    for (char element : ekspresiInfix.toCharArray()) {
+      if (Character.isLetterOrDigit(element)) {
+        output.append(element);
+      } else if (element == '(') {
+        stack.push(element);
+      } else if (element == ')') {
+        while (!stack.isEmpty() && stack.peek() != '(') {
+          output.append(stack.pop());
+        }
+        stack.pop();
+      } else {
+        while(!stack.isEmpty() && ekspresi(element) <= ekspresi(stack.peek())) {
+          output.append(stack.pop());
+        }
+        stack.push(element);
+      }      
+    }
+    while (!stack.isEmpty()) {
+      output.append(stack.pop());
+    }
+    return output.toString();
+  }
+
+  private static int ekspresi(char operator) {
+    switch (operator) {
+      case '+':
+      case '-':
+        return 0;
+      case '*':
+      case '/':
+        return 1;
+      case '^':
+        return 2;
+      default:
+        return -1;
+    }
+  }
+  public static void main(String[] args) throws Exception {
+
+  }
+}