Learn and practice the Cyclic Sort and Topological Sort patterns to solve coding interview problems.

Speedrun the Apple Coding Interview: Top Problems in Python

This module will focus on two advanced sorting algorithms, Cyclic Sort and Topological Sort. These are often used in coding interviews as a way to distinguish between average and outstanding candidates. Cyclic Sort may only be used if the input list meets certain criteria, and in those situations, it’s the most efficient sorting algorithm possible. Topological Sort utilizes graph-based algorithms to deal with partially ordered elements. This module teaches us to recognize whether either of these advanced sorting techniques is applicable, as well as how to use them to solve the given problem.

Advanced Sorting Techniques

# Statement

You are given an integer, `num_courses`, representing the total number of courses you need to complete, labeled from `0` to `num_courses - 1`.

You are also given a `prerequisites` array, where `prerequisites[i] = [a[i], b[i]]` indicates that you must take course `b[i]` first if you want to take the course `a[i]`. For example, the pair $[1, 0]$ indicates that to take course $1$, you have to first take course $0$.

Return TRUE if all of the courses can be finished. Otherwise, return FALSE.

**Constraints:**

- $1 \leq$ `num_courses` $\leq 1500$
- $0 \leq$ `prerequisites.length` $\leq 1000$
- `prerequisites[i].length` $= 2$
- $0 \leq$ `a[i]`, `b[i]` $<$ `num_courses`
- All the pairs `prerequisites[i]` are unique.


# Examples

What is the output if the following prerequisites and number of courses are given as input?

prerequisites = [[1, 0], [1, 2], [3, 1], [4, 1], [1, 4], [5, 1]]

num_courses = 6

What is the output if the following prerequisites and number of courses are given as input?
prerequisites = [[1, 0], [1, 2], [3, 1], [4, 1], [1, 4], [5, 1]]
num_courses = 6

What is the output if the following prerequisites and number of courses are given as input?

prerequisites = [[1, 0], [2, 1], [3, 2], [4, 3], [5, 4], [6, 2], [1, 6], [7, 1]]

num_courses = 8

What is the output if the following prerequisites and number of courses are given as input?
prerequisites = [[1, 0], [2, 1], [3, 2], [4, 3], [5, 4], [6, 2], [1, 6], [7, 1]]
num_courses = 8

What is the output if the following prerequisites and number of courses are given as input?

prerequisites = [[1, 0], [2, 1], [3, 1], [1, 4]]

num_courses = 5

What is the output if the following prerequisites and number of courses are given as input?
prerequisites = [[1, 0], [2, 1], [3, 1], [1, 4]]
num_courses = 5

Course Schedule

Course Schedule

## 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: `can_finish`\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 `can_finish` 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 `can_finish`.\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) `can_finish`. \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) `can_finish`. \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.

TwoDArray

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: MyCustomDS
class TwoDArray(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

        return json.loads(value)

    # input param contains return value of user program
    def traverse(self, input: any, type_of_arr: int) -> str:
        # TwoDArray your data structure 'input' to a json string representing a test case output
        return json.dumps(input)


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
 """
 from collections import deque

 def can_finish(num_courses, prerequisites):
 counter = 0
 if num_courses <= 0:
 return True

 inDegree = {i: 0 for i in range(num_courses)}
 graph = {i: [] for i in range(num_courses)}

 for edge in prerequisites:
 parent, child = edge[1], edge[0]
 graph[parent].append(child)
 inDegree[child] += 1

 sources = deque()
 for key in inDegree:
 if inDegree[key] == 0:
 sources.append(key)

 while sources:
 course = sources.popleft()
 counter += 1
 for child in graph[course]:
 inDegree[child] -= 1
 if inDegree[child] == 0:
 sources.append(child)

 return counter == num_courses

 # multiple outputs will be added at each index respectively for a single test_case
 expected = can_finish(self.__inputs[0], self.__inputs[1])
 actual_output = self.__actual_outputs[0]

 expected_outputs = [] 
 # store the 1st expected output that should be displayed to the user 
 expected_outputs.append(expected)

 if expected==actual_output:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

 return self.__edu_result
 return self.__edu_result

def can_finish(num_courses, prerequisites):

    # Replace this placeholder return statement with your code
    return False


PrivateTestCases.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : 2
    - 2 : [[1, 0], [0, 1]]
    output: 
    - 1 : False
  - name: "testcase 2"
    inputs:
    - 1: 5
    - 2: [[1, 0], [2, 1], [3, 2], [4, 3]]
    output:
    - 1 : True
  - name: "testcase 3"
    inputs:
    - 1: 5
    - 2: [[1, 0], [2, 1], [3, 2], [4, 3], [0, 4]]
    output:
    - 1 : False
  - name: "testcase 4"
    inputs:
    - 1: 41
    - 2: [[0, 1], [1, 2], [2, 3], [3, 4], [4, 5], [5, 6], [6, 7], [7, 8], [8, 9], [9, 10], [10, 11], [11, 12], [12, 13], [13, 14], [14, 15], [15, 16], [16, 17], [17, 18], [18, 19], [19, 20], [20, 21], [21, 22], [22, 23], [23, 24], [24, 25], [25, 26], [26, 27], [27, 28], [28, 29], [29, 30], [30, 31], [31, 32], [32, 33], [33, 34], [34, 35], [35, 36], [36, 37], [37, 38], [38, 39], [39, 40], [40, 0]]
    output:
    - 1 : False
  - name: "testcase 5"
    inputs:
    - 1: 5
    - 2: [[2, 0], [2, 1], [3, 2], [4, 2]]
    output:
    - 1 : True

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 the function
 store the result using the self.store_result(error_msg, status)
 return: void
 """
 min_courses = 1
 max_courses = 1500

 min_pre_req = 0
 max_pre_req = 1000

 n = self.__inputs[0]
 prerequisites = self.__inputs[1]

 def is_valid_2d_array(arr):
 if not isinstance(arr, list) or not all(isinstance(row, list) for row in arr):
 return False # Not a valid 2D array if it's empty or contains non-list elements
 
 for row in arr:
 if any(elem is None or isinstance(elem, bool) for elem in row):
 return False # Not a valid 2D array if any element in a row is None
 if not all(isinstance(elem, int) for elem in row):
 return False # Not a valid 2D array if any element in a row is not an integer
 
 return True # Valid 2D array
 # Inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_2d_array(prerequisites):
 self.store_result ("Input 2 is not a valid testcase.", ValidationFunction.FAIL)
 elif not (min_courses <= n <= max_courses):
 self.store_result("The number of courses 'num_courses' should be between 1 and 1500.", ValidationFunction.FAIL)
 elif not (min_pre_req <= len(prerequisites) <= max_pre_req):
 self.store_result("The number of 'prerequisites' should be between 0 and 1000.", ValidationFunction.FAIL)
 elif any(len(course_pair) != 2 for course_pair in prerequisites):
 self.store_result("Each course prerequisite pair in 'prerequisites' should have a length of 2.", ValidationFunction.FAIL)
 elif any(not isinstance(course_pair[0], int) or not isinstance(course_pair[1], int) or course_pair[0] is True or course_pair[0] is False or course_pair[1] is True or course_pair[1] is False for course_pair in prerequisites):
 self.store_result("Both elements of each course prerequisite pair should be integers.", ValidationFunction.FAIL)
 elif any(course_pair[0] < 0 or course_pair[0] >= n or course_pair[1] < 0 or course_pair[1] >= n for course_pair in prerequisites):
 self.store_result("The indices 'a' and 'b' should satisfy 0 ≤ a, b < n.", ValidationFunction.FAIL)
 elif len(prerequisites) != len(set(tuple(course_pair) for course_pair in prerequisites)):
 self.store_result("Course prerequisite pairs should be distinct.", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Course Schedule problem.

Cyclic Sort

Topological Sort

Conclusion

Course Schedule

Statement

Examples

Understand the problem

Figure it out!

Try it yourself