added problem of stack

Author: muskanmi

Stack/Postfix2.java

@@ -0,0 +1,89 @@
+package Stack;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+public class Postfix2 {
+
+    static int Prec(char ch)
+    {
+        switch (ch) {
+        case '+':
+        case '-':
+            return 1;
+  
+        case '*':
+        case '/':
+            return 2;
+  
+        case '^':
+            return 3;
+        }
+        return -1;
+    }
+
+    static String infixToPostfix(String exp)
+    {
+        // Initializing empty String for result
+        String result = new String("");
+  
+        // Initializing empty stack
+        Deque<Character> stack
+            = new ArrayDeque<Character>();
+  
+        for (int i = 0; i < exp.length(); ++i) {
+            char c = exp.charAt(i);
+  
+            // If the scanned character is an
+            // operand, add it to output.
+            if (Character.isLetterOrDigit(c))
+                result += c;
+  
+            // If the scanned character is an '(',
+            // push it to the stack.
+            else if (c == '(')
+                stack.push(c);
+  
+            // If the scanned character is an ')',
+            // pop and output from the stack
+            // until an '(' is encountered.
+            else if (c == ')') {
+                while (!stack.isEmpty()
+                       && stack.peek() != '(') {
+                    result += stack.peek();
+                    stack.pop();
+                }
+  
+                stack.pop();
+            }
+            
+            // An operator is encountered
+            else
+            {
+                while (!stack.isEmpty()
+                       && Prec(c) <= Prec(stack.peek())) {
+  
+                    result += stack.peek();
+                    stack.pop();
+                }
+                stack.push(c);
+            }
+        }
+  
+        // Pop all the operators from the stack
+        while (!stack.isEmpty()) {
+            if (stack.peek() == '(')
+                return "Invalid Expression";
+            result += stack.peek();
+            stack.pop();
+        }
+        
+        return result;
+    }
+    public static void main(String[] args) {
+        String exp = "a+b*(c^d-e)^(f+g*h)-i";
+        
+          // Function call
+        System.out.println(infixToPostfix(exp));
+    }
+}