Problem.java

/*
 * Click `Run` to execute the snippet below!
 */

import java.io.*;
import java.util.*;

/**
 * Design a calculator with the following features:
 *
 * * Add
 * * Subtract
 * * Multiply
 * * Divide
 * * Redo (Perform the most recent operation again)
 * * Undo (Revert the most recent operation)
 *
 * If you need more classes, simply define them inline.
 *
 * You can use System.out.println() to show the output of each operation. 
 * You do NOT need to use a CLI unless you feel you want to.
 */

class Solution {
  
  public static void main(String[] args) {
    // ADD 5 (Ouput should be 5)
    // ADD 5 (Output should be 10)
    // SUBTRACT 3 (Output should be 7)
    // UNDO (Output should be 10)
    // REDO (Output should be 15)
    // MULTIPLY by 2 (Output should be 30)
    // DIVIDE by 3 (Output should be 10)
  }
}

Solution.java

/*
 * Click `Run` to execute the snippet below!
 */

import java.io.*;
import java.math.BigDecimal;
import java.util.*;

/**
 * Design a calculator with the following features:
 *
 * * Add
 * * Subtract
 * * Multiply
 * * Divide
 * * Redo (Perform the most recent operation again)
 * * Undo (Revert the most recent operation)
 *
 * If you need more classes, simply define them inline.
 */

class Solution {
  public static void main(String[] args) {
    Stack<Operation> stack = new Stack<Operation>();

    System.out.println(add(BigDecimal.valueOf(5), stack));
    System.out.println(add(BigDecimal.valueOf(5), stack));
    System.out.println(subtract(BigDecimal.valueOf(3), stack));
    System.out.println(undo(stack));
    System.out.println(redo(stack));
    System.out.println(multiply(BigDecimal.valueOf(2), stack));
    System.out.println(divide(BigDecimal.valueOf(3), stack));
    System.out.println(clear(stack));
    System.out.println(undo(stack));
  }

  public static BigDecimal add(BigDecimal value, Stack<Operation> stack) {
    return calculateNewTotal(stack, new Operation("add", value));
  }

  public static BigDecimal subtract(BigDecimal value, Stack<Operation> stack) {
    return calculateNewTotal(stack, new Operation("subtract", value));
  }

  public static BigDecimal multiply(BigDecimal value, Stack<Operation>stack) {
    return calculateNewTotal(stack, new Operation("multiply", value));
  }

  public static BigDecimal divide(BigDecimal value, Stack<Operation> stack) {
    return calculateNewTotal(stack, new Operation("divide", value));
  }

  public static BigDecimal redo(Stack<Operation> stack) {
    if (stack.isEmpty()) {
      return BigDecimal.ZERO;
    }

    Operation lastOperation = stack.lastElement();

    return calculateNewTotal(stack, lastOperation);
  }

  public static BigDecimal undo(Stack<Operation> stack) {
    if (stack.isEmpty()) {
      return BigDecimal.ZERO;
    }

    stack.pop();

    if (stack.isEmpty()) {
      return BigDecimal.ZERO;
    }

    return stack.lastElement().total;
  }

  public static BigDecimal clear(Stack<Operation> stack) {
    stack.clear();

    return BigDecimal.ZERO;
  }

  private static BigDecimal calculateNewTotal(Stack<Operation> stack, Operation op) {
    BigDecimal currentTotal = BigDecimal.ZERO;
    BigDecimal newTotal = BigDecimal.ZERO;
    if (!stack.isEmpty()) {
      currentTotal = stack.lastElement().total;
    }

    switch (op.operation) {
      case "add":
        newTotal = currentTotal.add(op.value);
        break;
      case "subtract":
        newTotal = currentTotal.subtract(op.value);
        break;
      case "multiply":
        newTotal = currentTotal.multiply(op.value);
        break;
      case "divide":
        newTotal = currentTotal.divide(op.value);
        break;
    }

    // Update operation total before sending adding it.
    op.total = newTotal;

    stack.add(op);

    return newTotal;
  }

}

class Operation {
  public BigDecimal total;
  public String operation;
  public BigDecimal value;

  public Operation(BigDecimal total, String operation, BigDecimal value) {
    this.total = total;
    this.operation = operation;
    this.value = value;
  }

  public Operation(String operation, BigDecimal value) {
    this.operation = operation;
    this.value = value;
  }
}