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

Grokking Grind 75 in Python: 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
Design a custom queue, **MyQueue**, using only two stacks. Implement the **Push()**, **Pop()**, **Peek()**, and **Empty()** methods:

* **Void Push(int x):** Pushes element at the end of the queue.
* **Int Pop():** Removes and returns the element from the front of the queue.
* **Int Peek():** Returns the element at the front of the queue.
* **Boolean Empty():** Returns TRUE if the queue is empty. Otherwise FALSE.

You are required to only use the standard stack operations, which means that only the **Push()** to top, **Peek()** and **Pop()** from the top, **Size()**, and **Is Empty()** operations are valid.

> **Note:** In some interview questions, **Void Push(int x)** and **Int Pop()** might be referred to as **Void Enqueue(int x)** and **Int Dequeue()**, respectively.

**Constraints:**
* $1 <=$ `x` $<= 100$
* A maximum of $100$ calls can be made to **Push()**, **Pop()**, **Peek()**, and **Empty()**.
* The **Pop()** and **Peek()** methods will always be called on non-empty stacks.

# Understand the problem
Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

What is the output for the following series of commands?
```text
 push(10), push(20), pop(), peek()
```

What is the output for the following series of commands?
<pre><code class="language-text"> push(10), push(20), pop(), peek()
</code></pre>

What is the output for the following series of commands?
```text
 push(10), pop(), pop(), empty()
```

What is the output for the following series of commands?
<pre><code class="language-text"> push(10), pop(), pop(), empty()
</code></pre>

What is the output for the following series of commands?
```text
 push(10), push(20), push(30), pop(), peek(), pop()
```

What is the output for the following series of commands?
<pre><code class="language-text"> push(10), push(20), push(30), pop(), peek(), pop()
</code></pre>

Implement Queue Using Stacks

Implement Queue Using Stacks

# Figure it out!
We have a game for you to play. Rearrange the logical building blocks required to implement the **Push()** operation.

# Try it yourself
Implement your solution in `myqueue.py` in the following coding playground. We have provided some useful code templates in the other file that you may build on to solve this problem. 

from stack import Stack

class MyQueue(object):

    def __init__(self):
        # Write your code here
        pass

    def push(self, x):
        # Write your code here
        pass

    def pop(self):
        # Write your code here
        pass

    def peek(self):
        # Write your code here
        pass

    def empty(self):
        # Write your code here
        pass

class Stack:
    def __init__(self):
        self.stack_list = []

    def is_empty(self):
        return len(self.stack_list) == 0

    def top(self):
        if self.is_empty():
            return None
        return self.stack_list[-1]

    def size(self):
        return len(self.stack_list)

    def push(self, value):
        self.stack_list.append(value)

    def pop(self):
        if self.is_empty():
            return None
        return self.stack_list.pop()

# 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: `__init__, push, pop, peek, empty`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `Python`\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 `__init__, push, pop, peek, empty` 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 `__init__, push, pop, peek, empty`.\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) `__init__, push, pop, peek, empty`. \n- While calculating time complexity, include the time complexity of built-in Python 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) `__init__, push, pop, peek, empty`. \n- While calculating space complexity, include the space complexity of built-in Python 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.

OneDArray

import sys
import json

sys.path.append("../ds_v1")
from data_structure import DataStructure

# throw parsing errors for incorrect test case format
def parsingError(element_string, error_message):
    sys.stderr.write(f"Parsing Error: {element_string} {error_message}")
    exit()


# className: OneDArray
class OneDArray(DataStructure):
    def __init__(self):
        # constructor
        pass

    def __str__(self):
        # override print() for your data structure
        pass

    # return value will be passed to user program
    def construct(self, value: str, type_of_arr: int) -> any:
        # parse test case in json string 'value' to your desired data structure
        parse_input = json.loads(value)
        return parse_input

    # input param contains return value of user program
    def traverse(self, input: any, type_of_arr: int) -> str:
        # convert your data structure 'input' to a json string representing a test case output
        parse_output = json.dumps(input)
        return parse_output


def evaluate_results(self) -> object:
        """
        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
        """
        # Solution code
        # -------------------------------------------------------------------------------
        class Stack:
            def __init__(self):
                self.stack_list = []

            def is_empty(self):
                return len(self.stack_list) == 0

            def top(self):
                if self.is_empty():
                    return None
                return self.stack_list[-1]

            def size(self):
                return len(self.stack_list)

            def push(self, value):
                self.stack_list.append(value)

            def pop(self):
                if self.is_empty():
                    return None
                return self.stack_list.pop()

        class MyQueue(object):
            # constructor to initialize two stacks
            def __init__(self):
                self.stack1 = Stack()
                self.stack2 = Stack()

            def push(self, x):
                while not self.stack1.is_empty():
                    self.stack2.push(self.stack1.pop())
                self.stack1.push(x)

                while not self.stack2.is_empty():
                    self.stack1.push(self.stack2.pop())

            def pop(self):
                return self.stack1.pop()

            def peek(self):
                return self.stack1.top()

            def empty(self):
                return self.stack1.is_empty()

        def my_queue_test(input_1, input_2):
            # initialize a queue
                queue_obj = MyQueue()
                res = []
                # loop over all the commands
                for j in range(len(input_1)):
                    if input_1[j] == "push":
                        queue_obj.push(input_2[j])
                        res.append(None)
                    if input_1[j] == "pop":
                            val = queue_obj.pop()
                            res.append(val)
                    if input_1[j] == "empty":
                            res.append(queue_obj.empty())
                    if input_1[j] == "peek":
                            res.append(queue_obj.peek())
                return res

        # -------------------------------------------------------------------------------
        # multiple outputs will be added at each index respectively for a single test_case
        expected_outputs = [] 

        expected = my_queue_test(self.__inputs[0], self.__inputs[1])
        # store the 1st expected output that should be displayed to the user 
        expected_outputs.append(expected)

        actual = self.__actual_outputs[0]

        if expected == actual:
            self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
        else:
            self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

        return self.__edu_result

from myqueue import MyQueue

def my_queue_main(input_1, input_2):
    
    # initialize a queue
        queue_obj = MyQueue()
        res = []
        # loop over all the commands
        for j in range(len(input_1)):
            if input_1[j] == "push":
                # inputstr = input_1[j] + \
                    # "("+str(input_2[j])+")"
                # print("\t\t", inputstr, sep="")
                queue_obj.push(input_2[j])
                res.append(None)
            if input_1[j] == "pop":
                # inputstr = input_1[j] + \
                    # "("+str(input_2[j])+")"
                # print("\t\t", inputstr, "   returns ",
                      val = queue_obj.pop()
                      res.append(val)
            if input_1[j] == "empty":
                # inputstr = input_1[j] + \
                    # "("+str(input_2[j])+")"
                # print("\t\t", inputstr, " returns ",
                      res.append(queue_obj.empty())

            if input_1[j] == "peek":
                # inputstr = input_1[j] + \
                #     "("+str(input_2[j])+")"
                # print("\t\t", inputstr, "  returns ",
                      res.append(queue_obj.peek())
        return res




implement_queue_using_stacks.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : ["push", "push", "push", "pop", "peek", "empty"]
    - 2 : [9, 3, 1, null, null, null]
    output: 
    - 1 : [null, null, null, 9, 3, False]
  - name: "testcase 2"
    inputs:
    - 1: ["push", "push", "pop", "empty", "peek"]
    - 2: [10, 6, null, null, null]
    output:
    - 1 : [null, null, 10, False, 6]
  - name: "testcase 3"
    inputs:
    - 1: ["push", "push","push", "peek", "pop", "pop", "pop", "empty"]
    - 2: [1, 2, 3, null, null, null, null, null]
    output:
    - 1 : [null, null, null, 1, 1, 2, 3, True]
  - name: "testcase 4"
    inputs:
    - 1: ["push", "push", "peek", "pop", "empty"]
    - 2: [1, 2, null, null, null]
    output:
    - 1 : [null, null, 1, 1, False]
  - name: "testcase 5"
    inputs:
    - 1: ["push", "push", "peek", "pop", "pop", "empty", "push", "empty" ]
    - 2: [1, 2, null, null, null, null, 3, null]
    output:
    - 1 : [null, null, 1, 1, 2, True, null, False]
  

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
 """
 input_1 = self.__inputs[0]
 input_2 = self.__inputs[1]

 push_lower_bound = 1
 push_upper_bound = 100

 num_calls_lower_bound = 0
 num_calls_upper_bound = 100

 def check_param_values(input1):

 for i in range(0, len(input_1)):

 if input1[i] not in ["push", "pop", "peek", "empty"]:
 return "input method should only be from ['push' 'pop', 'peek', 'empty']"
 return True
 
 def equal_lengths(input1, input2):
 if len(input1) != len(input2):
 return "length of methods should be the same as length of params"
 return True
 
 def invalid_calls(input1):
 counter = 0
 for param in input1:

 if param == "push":
 counter += 1

 elif param == "pop" and counter > 0:
 counter -= 1

 elif param == "pop" and counter == 0:
 return "'pop' arguments are invalid: the stack is performing a pop operation in an empty stack"

 elif param == "peek" and counter == 0:
 return "'peek' arguments are invalid: the stack is performing a peek operation in an empty stack"

 return True
 
 def correct_corresponding_values(input1, input2):

 for i in range(len(input_1)):
 if input1[i] == "push" and type(input2[i]) != int:
 return "'push' arguments are invalid"

 elif input1[i] == "push" and not (push_lower_bound <= input2[i] <= push_upper_bound):
 return "'push' arguments are invalid: " + str(input2[i]) + " is not a valid value of type integer"

 elif input1[i] == "pop" and input2[i] != None:
 return "'pop' arguments are invalid"

 elif input1[i] == "peek" and input2[i] != None:
 return "'peek' arguments are invalid"

 elif input1[i] == "empty" and input2[i] != None:
 return "'empty' arguments are invalid"

 return True

 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if check_param_values(input_1) != True:
 self.store_result (check_param_values(input_1), ValidationFunction.FAIL)
 
 elif invalid_calls(input_1) != True:
 self.store_result(invalid_calls(input_1), ValidationFunction.FAIL)

 elif equal_lengths(input_1, input_2) != True:
 self.store_result(equal_lengths(input_1, input_2), ValidationFunction.FAIL)

 elif correct_corresponding_values(input_1, input_2) != True:
 self.store_result(correct_corresponding_values(input_1, input_2), ValidationFunction.FAIL)
 
 elif not (num_calls_lower_bound <= len(input_1[1:]) <= num_calls_upper_bound):
 self.store_result("expected 'Total number of calls' to have value from " + str(num_calls_lower_bound) + " to " + str(num_calls_upper_bound) + " only", ValidationFunction.FAIL)

 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Implement Queue Using Stacks problem.

In-Place Manipulation of a Linked List

Stacks

Matrices

Conclusion

Implement Queue Using Stacks

Statement

Examples

Understand the problem

Figure it out!

Try it yourself