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

Speedrun the Meta Coding Interview: Top Problems in Java

This module will focus on an advanced sorting algorithm, Topological Sort. These are often used in coding interviews as a way to distinguish between average and outstanding candidates.Topological Sort utilizes graph-based algorithms to deal with partially ordered elements. This module teaches us to recognize whether this advanced sorting techniques is applicable, as well as how to use them to solve the given problem.

Advanced Sorting Techniques



**Constraints:**

- $1 \leq$ `words.length` $\leq 10^3$
- $1 \leq$ `words[i].length` $\leq 20$
- All characters in `words[i]` are English lowercase letters.


# Examples

# Understand the problem

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


What is the output if the following list of words is given as input?

Select all that apply. 

["wrt", "wrf", "er", "ett", "rftt"]

What is the output if the following list of words is given as input?
Select all that apply.
[“wrt”, “wrf”, “er”, “ett”, “rftt”]

What is the output if the following list of words is given as input?

Select all that apply. 

["alpha", "bravo", "charlie", "delta"]

What is the output if the following list of words is given as input?
Select all that apply.
[“alpha”, “bravo”, “charlie”, “delta”]

What is the output if the following list of words is given as input?

Select all that apply. 

["jupyter", "ascending"]

What is the output if the following list of words is given as input?
Select all that apply.
[“jupyter”, “ascending”]

What is the output if the following list of words is given as input?

Select all that apply. 

["vanilla", "alpine", "algor", "port", "norm", "nylon", "ophellia", "hidden"]

What is the output if the following list of words is given as input?
Select all that apply.
[“vanilla”, “alpine”, “algor”, “port”, “norm”, “nylon”, “ophellia”, “hidden”]

Alien Dictionary

Alien Dictionary

# Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

# 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: `alienOrder`\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 `alienOrder` 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 `alienOrder`.\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) `alienOrder`. \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) `alienOrder`. \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:
 """
 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
 """

 #----------------

 def contains_elements(actual, expected):
 for i in expected:
 if i not in actual:
 return False
 return True
 #------------------ 
 def verifyResults( messages, key):
 # if len(messages) == 1:
 # return True

 keyMap = {}

 # for(int i = 0 i < key.size() i++):
 # keyMap[key[i]] = i
 # ] 
 if key:
 for i, val in enumerate(key):
 keyMap[val] = i
 
 for i in range( len(messages) - 1):
 for j in range (len(messages[i])):
 # If we do not find a mismatch letter between messages[i] and messages[i + 1],
 # we need to examine the case when messages are like ("educated", "educate").
 if j >= len(messages[i + 1]):
 return False

 if messages[i][j] != messages[i + 1][j]:
 if keyMap[messages[i][j]] > keyMap[messages[i + 1][j]]:
 return False
 
 else:
 # if we find the first different character and they are sorted,
 # then there's no need to check remaining letters
 break
 else:
 return False
 return True
 #------------------
 from collections import defaultdict, Counter, deque

 def alienOrder_test(words):
 # Step 0: Create data structures and find all unique letters.
 adj_list = defaultdict(set)
 # counts stores the number of unique letters
 counts = Counter({c: 0 for word in words for c in word})
 # Step 1: We need to populate adj_list and counts.
 # For each pair of adjacent words...
 outer = 0
 for word1, word2 in zip(words, words[1:]):
 outer += 1
 inner = 0
 for c, d in zip(word1, word2):
 inner += 1
 if c != d:
 if d not in adj_list[c]:
 adj_list[c].add(d)
 counts[d] += 1
 break
 else: # Check that second word isn't a prefix of first word.
 if len(word2) < len(word1):
 return ""

 # Step 2: We need to repeatedly pick off nodes with an indegree of 0.
 result = []
 queue = deque([c for c in counts if counts[c] == 0])
 m = 0
 while queue:
 m += 1
 c = queue.popleft()
 result.append(c)
 n = 0
 for d in adj_list[c]:
 n += 1
 counts[d] -= 1
 if counts[d] == 0:
 queue.append(d)

 # If not all letters are in result, that means there was a cycle and so
 # no valid ordering. Return "".
 if len(result) < len(counts):
 return ""
 # Otherwise, convert the ordering we found into a string and return it.
 return "".join(result)
 
 # multiple outputs will be added at each index respectively for a single test_case
 expected_outputs = []
 
 # store the 1st expected output that should be displayed to the user 
 # input = json.loads(self.__inputs)
 expected = alienOrder_test(self.__inputs[0])
 actual = self.__actual_outputs[0]


 if actual and expected and len(expected) != len(actual):
 expected_outputs.append(expected)
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)
 elif not actual and not expected and expected == actual: 
 expected_outputs.append(expected) 
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS) 
 elif actual and len(expected) == len(actual) and contains_elements(actual, expected) and verifyResults(self.__inputs, actual):
 expected_outputs.append(actual)
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else: 
 expected_outputs.append(expected)
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL) 


 return self.__edu_result

import java.util.*;

public class Main{
 public static String alienOrder(List<String> words) {
 
 // Replace this placeholder return statement with your code
 return "";
 }
}

PrivateTestCases.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : ["xro", "xma","per","prt","oxh","olv"]

  - name: "testcase 2"
    inputs:
    - 1: ["o", "l", "m", "s"]

  - name: "testcase 3"
    inputs:
    - 1: ["mdx", "mars", "avgd", "dkae"]

  - name: "testcase 4"
    inputs:
    - 1: ["mdxok", "mrolw", "mroqs", "kptz", "klr", "klon", "zvef", "zrsu", "zzs", "orm", "oqt"]

  - name: "testcase 5"
    inputs:
    - 1: ["m", "mx", "mxe", "mxer", "mxerl", "mxerlo", "mxerlos", "mxerlost", "mxerlostr", "mxerlostrpq", "mxerlostrp"]

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_word_count = 1
 max_word_count = 1000
 min_word_length = 1
 max_word_length = 20

 words = self.__inputs[0]
 def is_valid_1d_array(arr):
 if not isinstance(arr, list):
 return False # Not a valid 1D array if it's not a list
 
 return all(isinstance(elem, str) and not isinstance(elem, bool) for elem in arr)
 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_1d_array(words):
 self.store_result(("Invalid test case."), ValidationFunction.FAIL)
 elif not (min_word_count <= len(words) <= max_word_count):
 self.store_result("The number of words should be between 1 and 10^3.", ValidationFunction.FAIL)
 elif any(not isinstance(word, str) for word in words):
 self.store_result("Each word should be a string.", ValidationFunction.FAIL)
 elif any(not (min_word_length <= len(word) <= max_word_length) for word in words):
 self.store_result("Each word should have a length between 1 and 20.", ValidationFunction.FAIL)
 elif any(not word.islower() for word in words):
 self.store_result("Each word should be a lowercase English word.", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Alien Dictionary problem.

Topological Sort

Conclusion

Alien Dictionary

Statement

Examples

Understand the problem

Figure it out!

Try it yourself