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

Speedrun the LinkedIn Coding Interview: Top JavaScript Problems

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 
We are given an integer number, `n`, representing the number of functions running in a single-threaded CPU, and an execution log, which is essentially a list of strings. Each string has the format `{function id}:{"start" | "end"}:{timestamp}`, indicating that the function with `function id` either started or stopped execution at the time identified by the `timestamp` value. Each function has a unique ID between $0$ and $n-1$. Compute the exclusive time of the functions in the program.

> **Note:** The exclusive time is the sum of the execution times for all the calls to a specific function.
 
**Constraints:**
* $1 \leq$ `n` $\leq 100$
* $1 \leq$ `logs.length` $\leq 500$
* $0 \leq$ `function id` $<$ `n`
* $0 \leq$ `timestamp` $\leq 10^3$
* No two start events and two end events will happen at the same `timestamp`.
* Each function has an `end` log entry for each `start` log entry.
 
# Examples 
Each function is identified in the logs by a `function id`. Each log entry is formatted in the following way: 
```
{function id}:{"start" | "end"}:{timestamp}
```

The above log entries indicate that three functions with IDs `0`, `1`, and `2` are executed as shown in the following figure. The function with ID `0` started execution at time `0` and ended at time `8`. However, since this is a single-threaded CPU, only one function can run at a time. So, when the function with ID `1` starts execution at time `2`, the function with ID `0` is preempted. As soon as the function with ID `1` stops execution at time `3`, the function with ID `2` starts execution, so the function with ID `0` still remains preempted. Finally, the function with ID `2` ends execution at time `7` and the function with ID `0` resumes and finishes execution at time `8`.


Our task is to return the total time for which each function ran. For example, the function with ID `0` ran for a total of `3` time units.

Here are some example inputs and the corresponding expected output:

# 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:



Given the following logs and the value of n, what will be the output?

logs $=$ ['0:start:0', '1:start:6', '1:end:6', '0:end:7']

n $=$ 2

Given the following logs and the value of n, what will be the output?
logs <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo></mrow><annotation encoding="application/x-tex">=</annotation></semantics></math>= [‘0:start:0’, ‘1:start:6’, ‘1:end:6’, ‘0:end:7’]
n <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo></mrow><annotation encoding="application/x-tex">=</annotation></semantics></math>= 2

Given the following logs and value of $n$, what will be the output?

logs $=$ ['0:start:0', '1:start:3', '1:end:6', '0:end:10']

n $=$ 2

Given the following logs and value of <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>n</mi></mrow><annotation encoding="application/x-tex">n</annotation></semantics></math>n, what will be the output?
logs <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo></mrow><annotation encoding="application/x-tex">=</annotation></semantics></math>= [‘0:start:0’, ‘1:start:3’, ‘1:end:6’, ‘0:end:10’]
n <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo></mrow><annotation encoding="application/x-tex">=</annotation></semantics></math>= 2

Exclusive Execution Time of Functions

Exclusive Execution Time of Functions


# 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 `main.js` in the following coding playground. The supporting code template provided in `log.js` is meant to assist in developing your solution to the problem.

export class Log {
    constructor(content) {
        content = content.replace(" ", "");
        this.strs = content.split(":");
        this.id = parseInt(this.strs[0]);
        this.isStart = this.strs[1] == "start";
        this.time = parseInt(this.strs[2]);
    }
}

class Event:
    def __init__(self, content):
        content = content.replace(' ', '')
        content = content.split(":")
        self.id = int(content[0])
        self.is_start = content[1] == "start"
        self.time = int(content[2])

# Creating a stack
def create_stack():
    stack = []
    return stack


# Checking if stack is empty or not
def check_empty(stack):
    return len(stack) == 0


# Push function adds items into the stack
def push(stack, item):
    stack.append(item)
    print("Pushed item: " + item)


# Pop function removes an element from the stack
def pop(stack):
    if (check_empty(stack)):
        return "Stack is empty"

    return stack.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: `exclusiveTime`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `JavaScript`\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 `exclusiveTime` 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 `exclusiveTime`.\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) `exclusiveTime`. \n- While calculating time complexity, include the time complexity of built-in JavaScript 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) `exclusiveTime`. \n- While calculating space complexity, include the space complexity of built-in JavaScript 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
        """

        # Log class
        class Log:
            def __init__(self, content):
                content = content.replace(' ', '')
                content = content.split(":")
                self.id = int(content[0])
                self.is_start = content[1] == "start"
                self.time = int(content[2])
        
        # Solution
        def exclusive_time(n, logs):
            logs_stack = []
            result = [0]*n

            for content in logs:
                logs = Log(content)
                if logs.is_start:
                    logs_stack.append(logs)
                else:
                    top = logs_stack.pop()
                    result[top.id] += (logs.time - top.time + 1)
                    if logs_stack:
                        result[logs_stack[-1].id] -= (logs.time - top.time + 1)
            return result
        
        # Expected output
        expected = exclusive_time(self.__inputs[0], self.__inputs[1])
        
        # Actual output
        actual = self.__actual_outputs[0]

        # 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 
        expected_outputs.append(expected)

        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 { Log } from "./log.js";

export function exclusiveTime(n, events) {

    // Replace this placeholder return statement with your code
    return []
}

exclusive_time_testcases.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : 1
    - 2 : ["0:start:0","0:start:2","0:end:5","0:start:6","0:end:6","0:end:7"]
    output: 
    - 1 : [8]

  - name: "testcase 2"
    inputs:
    - 1 : 2
    - 2 : ["0:start:0", "1:start:3", "1:end:6", "0:end:10"]
    output: 
    - 1 : [7, 4]

  - name: "testcase 3"
    inputs:
    - 1 : 3
    - 2 : ["0:start:0","0:end:0","1:start:1","1:end:1","2:start:2","2:end:2","2:start:3","2:end:3"]
    output: 
    - 1 : [1, 1, 2]

  - name: "testcase 4"
    inputs:
    - 1 : 5
    - 2 : ["0:start:0","1:start:5","1:end:9","4:start:10","2:start:13","2:end:15","3:start:16","3:end:18","4:end:21","0:end:22"]
    output: 
    - 1 : [6, 5, 3, 3, 6]

  - name: "testcase 5"
    inputs:
    - 1 : 6
    - 2 : ["0:start:0","1:start:5","2:start:8","3:start:12","4:start:15","5:start:19","5:end:22","4:end:24","3:end:27","2:end:32","1:end:35","0:end:36"]
    output: 
    - 1 : [6, 6, 9, 6, 6, 4]

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 self.store_result(error_msg, status).
 return: void
 """
 def is_valid_logs_array(logs, n):
 if not isinstance(logs, list):
 return False # Logs must be a list
 
 timestamps = set()
 start_count = {} # To match start and end counts for each function
 end_count = {}
 last_timestamp = -1 # To validate non-decreasing timestamps
 
 for log in logs:
 # Check if log is a string
 if not isinstance(log, str):
 return False
 
 parts = log.split(":")
 # Check if log has exactly 3 parts: function_id, 'start'/'end', and timestamp
 if len(parts) != 3:
 return False
 
 func_id, status, timestamp = parts
 
 # Validate function_id format
 if not func_id.isdigit() or not (0 <= int(func_id) < n):
 return False
 func_id = int(func_id)
 
 # Validate status format
 if status not in ("start", "end"):
 return False
 
 # Validate timestamp format
 if not timestamp.isdigit() or not (0 <= int(timestamp) <= 10**3):
 return False
 timestamp = int(timestamp)
 
 # Check for non-decreasing timestamps
 if timestamp < last_timestamp:
 return False # Timestamps are not non-decreasing
 last_timestamp = timestamp
 
 # Check for duplicate timestamps for the same status
 if (status, timestamp) in timestamps:
 return False # Duplicate start or end event at the same timestamp
 timestamps.add((status, timestamp))
 
 # Update start and end counts
 if status == "start":
 start_count[func_id] = start_count.get(func_id, 0) + 1
 elif status == "end":
 end_count[func_id] = end_count.get(func_id, 0) + 1
 
 # Check if start and end counts match for each function
 for func_id in start_count:
 if start_count.get(func_id, 0) != end_count.get(func_id, 0):
 return False # Mismatch in start and end log entries
 
 return True
 
 n = self.__inputs[0] # Number of functions
 logs = self.__inputs[1] # Execution log
 
 # Validate n
 if not isinstance(n, int) or not (1 <= n <= 100):
 self.store_result("'n' must be an integer between 1 and 100.", ValidationFunction.FAIL)
 # Validate logs
 elif not is_valid_logs_array(logs, n):
 self.store_result("'logs' must be a list of strings in the exact format '{function id}:start|end:{timestamp}', with valid values, matching constraints, and non-decreasing timestamps.", ValidationFunction.FAIL)
 # Validate logs length
 elif not (1 <= len(logs) <= 500):
 self.store_result("'logs' length must be between 1 and 500.", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Exclusive Time of Functions problem.

In-place Manipulation of a Linked List

Stacks

Matrices

Conclusion

Exclusive Execution Time of Functions

Statement

Examples

Understand the problem

Figure it out!

Try it yourself