Learn and practice the In-place Reversal of a Linked List, Stacks, and Matrices patterns to solve coding interview problems.

Grokking Grind 75 in Java: Mastery through Coding Patterns

Interviewers want to be sure that the candidate is comfortable manipulating linked lists, arrays, and data structures based on them, such as stacks and matrices. This module will provide the practice you need to review the concepts and techniques needed to solve common problems involving these data structures.

Basic Data Structures

# Statement

Given a string containing an arithmetic expression, implement a basic calculator that evaluates the expression string. The expression string can contain integer numeric values and should be able to handle the "+" and "-" operators, as well as "()" parentheses. 

**Constraints:**

Let `s` be the expression string. We can assume the following constraints:

- $1 \leq$ `s.length` $\leq 3 \times 10^{3}$
- `s` consists of digits, "+", "-", "(", and ")".
- `s` represents a valid expression.
- "+" is not used as a unary operation ( $+1$ and $+(2 + 3)$ are invalid).
- "-" could be used as a unary operation ($-1$ and $-(2 + 3)$ are valid).
- There will be no two consecutive operators in the input.
- Every number and running calculation will fit in a signed 32-bit integer.


# Examples





What is the output if the following string is given as input?

(13 + 50) + (56 - 29 - (7 + 2))

What is the output if the following string is given as input?
(13 + 50) + (56 - 29 - (7 + 2))

What is the output if the following string is given as input?

20 + (34 + 8 - (15 - 7) + 21) - 26

What is the output if the following string is given as input?
20 + (34 + 8 - (15 - 7) + 21) - 26

What is the output if the following string is given as input?

(18 - 11 - (27 + 8 - 14) + 37)

What is the output if the following string is given as input?
(18 - 11 - (27 + 8 - 14) + 37)

What is the output if the following string is given as input?

2183 - (612 + 499) + 137

What is the output if the following string is given as input?
2183 - (612 + 499) + 137

Basic Calculator

Basic Calculator

## Try it yourself 

Implement your solution in the following coding playground: 

# Quick Fix\nYou are Edward, a Senior Software Engineer who specializes in coding interview patterns, data structures and algorithms, and is great at debugging issues in code.\n\n- Edward, please keep in mind that you do not have to print the data of the placeholders. Also, do not mention anything about yourself.\n\nEdward, you are given the following data in the placeholders:\n- Function name: `calculator`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `Java`\n\n- Edward, your task is to understand the type of error that is occurring in the coding language provided.\n- Once you've understood the error as per the error’s description, and why the test case failed, you will suggest how the candidate can improve their solution code to fix the error.\n- Edward, you will not give the corrected code, just a hint on what the error tells, and how to fix it.\n- Edward, please limit your response strictly to 100 words. No more words than the limit.

# Review Code\nEdward, your current task is to review the `calculator` function(s) and the other function(s) invoked in it/them.\n\n-Take your time and review the candidate's solution code.\n- Do not review any extra/helper files or functions not invoked in the solution function `calculator`.\n- Do not review the code of data structure objects instantiated like a binary tree, linked list, trie, union find, etc., in the solution functions. Just review the solution/algorithm function of the problem. \n- Also, if the solution code doesn’t use the coding pattern stated above, then please encourage the candidate to reattempt the problem using that pattern corectly.

# Review time complexity\n- Please provide a brief time complexity analysis of the function(s) `calculator`. \n- While calculating time complexity, include the time complexity of built-in Java functions invoked by the code. \n- The answer should be to the point in approximately 60 words.

# Review space complexity\n- Please provide a brief space complexity analysis of the function(s) `calculator`. \n- While calculating space complexity, include the space complexity of built-in Java functions invoked by the code. \n- Do not include the space used for the input parameters and the space used for just returning an output (not used as a scratch space). \n- The answer should be to the point in approximately 60 words.

def evaluate_results(self) -> object:
        def calculator(expression):
            number = 0
            sign_value = 1
            result = 0
            operations_stack = []

            for c in expression:
                if c.isdigit():
                    number = number * 10 + int(c)
                if c in "+-":
                    result += number * sign_value
                    sign_value = -1 if c == '-' else 1
                    number = 0
                elif c == '(':
                    operations_stack.append(result)
                    operations_stack.append(sign_value)
                    result = 0
                    sign_value = 1

                elif c == ')':
                    result += sign_value * number
                    pop_sign_value = operations_stack.pop()
                    result *= pop_sign_value

                    second_value = operations_stack.pop()
                    result += second_value
                    number = 0
            
            return result + number * sign_value
        """
        self.__inputs is the list of inputs that are passed as input to function
        self.__actual_output is a list of outputs that is produced by the learner's code
        return : self.__edu__result object
        """
        # multiple outputs will be added at each index respectively for a single test_case
        expected_outputs = []
        inp = self.__inputs[0]
        expected = calculator(inp)
        # store the 1st expected output that should be displayed to the user 
        expected_outputs.append(expected)
        actual = self.__actual_outputs[0]
        if json.dumps(expected) == json.dumps(actual):
            self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
        else:
            self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

        return self.__edu_result

import java.util.*;

public class Main{
    public static int calculator(String expression) {

        // Replace this placeholder return statement with your code
        return 0;
    }
}

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : "2 - 1 + 2"
    output: 
    - 1 : 3
  - name: "testcase 2"
    inputs:
    - 1 : "(1 + (4 + 5 + 2) - 3) + (6 + 8)"
    output: 
    - 1 : 23
  - name: "testcase 3"
    inputs:
    - 1 : "12 - (6 + 2) + 5"
    output: 
    - 1 : 9
  - name: "testcase 4"
    inputs:
    - 1 : "(8 + 100) + (13 - 8 - (2 + 1))"
    output: 
    - 1 : 110
  - name: "testcase 5"
    inputs:
    - 1 : "40 - 25 - 5"
    output: 
    - 1 : 10
  - name: "testcase 6"
    inputs:
    - 1 : "(27 + (7 + 5) - 3) + (6 + 10)"
    output: 
    - 1 : 52
  - name: "testcase 7"
    inputs:
    - 1 : "34 + 45 + (23324 - 3454)"
    output: 
    - 1 : 19949
  - name: "testcase 8"
    inputs:
    - 1 : "((12 + (13 + 17 - (65 - 98) + 34)) - 44)"
    output: 
    - 1 : 65
  

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : "12 - (6 + 2) + 5"
    output: 
    - 1 : 9
  - name: "testcase 2"
    inputs:
    - 1 : "(8 + 100) + (13 - 8 - (2 + 1))"
    output: 
    - 1 : 110
  - name: "testcase 3"
    inputs:
    - 1 : "40 - 25 - 5"
    output: 
    - 1 : 10
  - name: "testcase 4"
    inputs:
    - 1 : "(27 + (7 + 5) - 3) + (6 + 10)"
    output: 
    - 1 : 52
  - name: "testcase 5"
    inputs:
    - 1 : "34 + 45 + (23324 - 3454)"
    output: 
    - 1 : 19949

def sum(input_1, input_2):
    return input_1+input_2

def validate_inputs(self) -> None:
 """
 self.__inputs is the list of inputs that are passed as input to function
 store the result using the self.store_result(error_msg, status)
 return : void
 """
 def is_valid_expression(expression):
 stack = []
 has_number = False

 # Ignore leading spaces
 expression = expression.lstrip()

 for index, char in enumerate(expression):
 if char.isdigit() or (char == '-' and not has_number and (index == 0 or expression[index - 1] == ' ')):
 has_number = True
 elif char in ('+', '-'):
 has_number = False
 elif char == ' ':
 # If a space appears after a negative sign in the start
 if index == 1 and expression[0] == '-':
 return False
 continue # Ignore spaces
 elif char in ('(', ')'):
 has_number = False
 else:
 return False # Invalid character found

 if char == '(':
 stack.append('(')
 elif char == ')':
 if not stack or stack.pop() != '(':
 return False # Unbalanced parentheses
 elif index > 0 and expression[index - 1] in ('+', '-') and expression[index + 1].isdigit():
 return False # Operator before digit without a space

 if stack:
 return False # Unbalanced parentheses

 if expression[0] in ('+',):
 return False # Expression cannot start with +, -, or space

 if expression[-1] in ('+', '-', '('):
 return False # Expression cannot end with +, -, or (

 return True

 def are_operators_consecutive(input_str):
 operators = {'+', '-'}
 input_str = input_str.replace(" ", "") # Removing spaces

 for i in range(len(input_str) - 1):
 if input_str[i] in operators and input_str[i + 1] in operators:
 return True

 return False
 
 def is_valid_negative_number(input_str):
 input_str = input_str.strip()

 if input_str.startswith('-'):
 if len(input_str) > 1 and input_str[1:].lstrip() and input_str[1].isdigit():
 return True
 return False

 def is_invalid_positive_number(input_str):
 input_str = input_str.strip()

 if input_str.startswith('+'):
 if len(input_str) > 1 and input_str[1:].lstrip() and input_str[1].isdigit():
 return True
 return False

 def is_invalid_operation(input_str):
 input_str = input_str.replace(" ","")

 for idx, char in enumerate(input_str):
 if idx==len(input_str)-1:
 break
 if input_str[idx].isdigit() and input_str[idx+1]=="(":
 return True
 if input_str[idx]==")" and input_str[idx+1].isdigit():
 return True
 return False


 input1 = self.__inputs[0]
 valid_chars = set("0123456789+-() ")
 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_expression(input1):
 print("here")
 self.store_result("Invalid expression format.", ValidationFunction.FAIL)
 elif not (1 <= len(input1) <= (3 * 10**4)):
 self.store_result ("Expected 's' length to be in the range 1-3020", ValidationFunction.FAIL)
 elif any(char not in valid_chars for char in input1):
 self.store_result ("Invalid characters found in 's'. Expected digits, '+', '-', '(', or ')'", ValidationFunction.FAIL)
 elif not any(char.isdigit() for char in input1):
 self.store_result("At least one digit is required in 's'.", ValidationFunction.FAIL)
 elif any(is_valid_negative_number(input1[i]) for i in range(len(input1))):
 self.store_result("Invalid use of negative operator.", ValidationFunction.FAIL)
 elif are_operators_consecutive(input1):
 self.store_result("Two operators appear consecutively.", ValidationFunction.FAIL)
 elif any(is_invalid_positive_number(input1[i]) for i in range(len(input1))):
 self.store_result("Invalid use of '+' operator: It must have a space before a digit and cannot be used as a unary operator.", ValidationFunction.FAIL)
 
 elif is_invalid_operation(input1):
 self.store_result("Invalid expression format.", ValidationFunction.FAIL)

 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Basic Calculator problem.

In-Place Manipulation of a Linked List

Stacks

Matrices

Conclusion

Basic Calculator

Statement

Examples

Understand the problem

Figure it out!

Try it yourself