Merge pull request #614 from tiger3768/stjava

Add : Balanced Brackets and InfixToPrefix program in Java

Java/Stack/BalancedBrackets.java

@@ -0,0 +1,32 @@
+//Program to check for Balanced Brackets in Java
+
+import java.util.Stack;
+
+public class BalancedBrackets {
+    public static boolean isBalanced(String expression) {
+        Stack<Character> bracketStack = new Stack<>();
+        int n = expression.length();
+        for(int i = 0 ; i < n ; i++){
+            char bracket = expression.charAt(i);
+            if(isNotB(bracket)) continue;
+            if(isOpeningB(bracket)) bracketStack.push(bracket);
+            else{
+                if(!bracketStack.isEmpty() && validOpeningB(bracket) != bracketStack.peek()) return false;
+                else bracketStack.pop();
+            }
+        }
+        return bracketStack.isEmpty();
+    }
+    static boolean isNotB(char bracket){
+        return !("(){}[]".contains(bracket + ""));
+    }
+    static boolean isOpeningB(char bracket){
+        return "({[".contains(bracket + "");
+    }
+    static char validOpeningB(char bracket){
+        if(bracket == ')') return '(';
+        if(bracket == '}') return '{';
+        if(bracket == ']') return '[';
+        return '\0';
+    }
+}

Java/Stack/InfixToPrefix.java

@@ -0,0 +1,42 @@
+//Java Program to convert Infix Expression to Prefix
+import java.util.Stack;
+
+public class InfixToPrefix {
+    public static void main(String[] args) {
+        System.out.println("1+2+3 -> " + infix2Prefix("1+2+3"));
+        System.out.println("A^B-(C+D) -> " + infix2Prefix("A^B-(C+D)"));
+        System.out.println("(P/L+(F*J) -> " + infix2Prefix("(P/L+(F*J)"));
+    }
+    public static String infix2Prefix(String expression){
+        if(!BalancedBrackets.isBalanced(expression)) return "Invalid Expression";
+        Stack<Character> operatorStack = new Stack<>();
+        StringBuilder prefixExpression = new StringBuilder();
+        int n = expression.length();
+        for(int i = n - 1 ; i >= 0 ; i--){
+            char scanned = expression.charAt(i);
+            if(Character.isLetterOrDigit(scanned)) prefixExpression.append(scanned);
+            else if(scanned == ')') operatorStack.push(scanned);
+            else if(scanned == '('){
+                while(!operatorStack.isEmpty() && operatorStack.peek() != ')'){
+                    prefixExpression.append(operatorStack.pop());
+                }
+                if(!operatorStack.isEmpty()) operatorStack.pop();
+            }
+            else if(operatorStack.isEmpty() || operatorStack.peek() == ')') operatorStack.push(scanned);
+            else{
+                while(!operatorStack.isEmpty() && precedence(operatorStack.peek()) < precedence(scanned)){
+                    prefixExpression.append(operatorStack.pop());
+                }
+                operatorStack.push(scanned);
+            }
+        }
+        while(!operatorStack.isEmpty()) prefixExpression.append(operatorStack.pop());
+        return prefixExpression.reverse().toString();
+    }
+    static int precedence(char operator){
+        if(operator == '^') return 3;
+        if(operator == '*' || operator == '/') return 2;
+        if(operator == '+' || operator == '-') return 1;
+        return 0;
+    }
+}