Question

Using Java 8. Write a program that reads an expression in postfix notation, builds the expression tree and prints the expression in prefix and infix notation and evaluates the expression. (Hint use a stack)

Answer 1

import java.util.*; class Solution {     static boolean isOperand(char x) {     return (x >= 'a' && x <= 'z') ||             (x >= 'A' && x <= 'Z'); }    // Get Infix for a given postfix // expression static String getInfix(String exp) {     Stack<String> s = new Stack<String>();        for (int i = 0; i < exp.length(); i++)     {         // Push operands         if (isOperand(exp.charAt(i)))         {         s.push(exp.charAt(i) + "");         }            // We assume that input is         // a valid postfix and expect         // an operator.         else         {             String op1 = s.peek();             s.pop();             String op2 = s.peek();             s.pop();             s.push("(" + op2 + exp.charAt(i) +                     op1 + ")");         }     }        // There must be a single element     // in stack now which is the required     // infix.     return s.peek(); }     static boolean isOperator(char x)     {         switch (x) {         case '+':         case '-':         case '/':         case '*':             return true;         }         return false;     }     // Convert postfix to Prefix expression     static String postToPre(String post_exp)     {         Stack<String> s = new Stack<String>();         // length of expression         int length = post_exp.length();         // reading from right to left         for (int i = 0; i < length; i++) {             // check if symbol is operator             if (isOperator(post_exp.charAt(i))) {                 // pop two operands from stack                 String op1 = s.peek();                 s.pop();                 String op2 = s.peek();                 s.pop();                 // concat the operands and operator                 String temp                     = post_exp.charAt(i) + op2 + op1;                 // Push String temp back to stack                 s.push(temp);             }             // if symbol is an operand             else {                 // push the operand to the stack                 s.push(post_exp.charAt(i) + "");             }         }         // concatenate all strings in stack and return the         // answer         String ans = "";         for (String i : s)             ans += i;         return ans;     }     // Driver Code     public static void main(String args[])     {         String post_exp = "ABC/-AK/L-*";         // Function call         System.out.println("Prefix : "                            + postToPre(post_exp));         System.out.println( "Infix : "+getInfix(post_exp));     } }