What is the time complexity of Remove Comments?

The optimal solution has a time complexity of O(N).

What data structures are used to solve Remove Comments?

This problem uses Array, String. Understanding these concepts is essential for solving this Medium problem efficiently.

What are the key insights for solving Remove Comments?

Insight 1: The key is to maintain a state variable `in_block_comment` to track whether we are currently inside a block comment. This is crucial for correctly handling multi-line comments. Insight 2: We need to iterate through each line and each character, checking for comment delimiters (`//` and `/*`). The order of these checks is important because we want to identify comments based on their first effective occurrence.

Which companies ask LeetCode 722. Remove Comments in interviews?

This problem is frequently asked at Hudson River Trading.

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Remove Comments - LeetCode 722 Solution

Remove Comments - Complete Solution Guide

Remove Comments is LeetCode problem 722, a Medium level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

Given a C++ program, remove comments from it. The program source is an array of strings source where source[i] is the i th line of the source code. This represents the result of splitting the original source code string by the newline character '\n' . In C++, there are two types of comments, line comments, and block comments. The string "//" denotes a line comment, which represents that it and the rest of the characters to the right of it in the same line should be ignored. The string "/*" denot

Detailed Explanation

The problem requires you to remove C++-style comments (both single-line `//` and multi-line `/* ... */`) from a given source code represented as a list of strings, where each string is a line of code. The goal is to produce a new list of strings, where each string is a line of code with comments removed. Empty lines resulting from comment removal should not be included in the output. The most important detail is to handle the precedence between line and block comments, and to account for block comments potentially spanning multiple lines. The input source code is guaranteed to be valid, with no unmatched block comment delimiters and no special characters that would interfere with comment parsing.

Solution Approach

The provided solution iterates through each line of the source code and character by character within each line. A boolean variable `in_block_comment` keeps track of whether we are currently inside a multi-line comment. If not inside a block comment, it checks for `//` or `/*`. If a `//` is found, the rest of the line is skipped. If `/*` is found, `in_block_comment` is set to `true`. If inside a block comment, the code looks for `*/` to signal the end of the block comment and sets `in_block_comment` to `false`. All non-comment parts are appended to a `current_line_build` string. Finally, the built-up line is added to the `result` list if it's not empty and not inside a block comment at the end of processing that line.

Step-by-Step Algorithm

Step 1: Initialize an empty list `result` to store the lines without comments and a boolean variable `in_block_comment` to `false`.
Step 2: Iterate through each line in the input `source` list.
Step 3: For each line, check if we are currently in a block comment. If not, reset the `current_line_build` string/StringBuilder.
Step 4: Iterate through each character in the current line.
Step 5: If `in_block_comment` is true, check for the end of the block comment (`*/`). If found, set `in_block_comment` to `false` and move the index past `*/`. Otherwise, increment the index to skip the character.
Step 6: If `in_block_comment` is false, check for a line comment (`//`). If found, break the inner loop, skipping the rest of the line.
Step 7: If `in_block_comment` is false, check for the start of a block comment (`/*`). If found, set `in_block_comment` to `true` and move the index past `/*`.
Step 8: If none of the above conditions are met, append the current character to `current_line_build`.
Step 9: After processing all characters in the line, if `in_block_comment` is false and `current_line_build` is not empty, add the line to the `result` list.
Step 10: Return the `result` list.

Key Insights

Insight 1: The key is to maintain a state variable `in_block_comment` to track whether we are currently inside a block comment. This is crucial for correctly handling multi-line comments.
Insight 2: We need to iterate through each line and each character, checking for comment delimiters (`//` and `/*`). The order of these checks is important because we want to identify comments based on their first effective occurrence.
Insight 3: The code should handle cases where a block comment begins on one line and ends on another. The implicit newline character deletion is an important aspect addressed by maintaining the `in_block_comment` state.

Complexity Analysis

Time Complexity: O(N)

Space Complexity: O(N)

Topics

This problem involves: Array, String.

Companies

Asked at: Hudson River Trading.

Remove Comments | LeetCode 722

Medium

Array String

Time: O(N)Space: O(N)

Solution Code

Available in:

import java.util.ArrayList;
import java.util.List;

class Solution {
    public List<String> removeComments(String[] source) {
        List<String> result = new ArrayList<>();
        boolean in_block_comment = false;
        StringBuilder current_line_build = new StringBuilder();

        for (String line : source) {
            int i = 0;
            
            if (!in_block_comment) {
                current_line_build = new StringBuilder();
            }

            while (i < line.length()) {
                if (in_block_comment) {
                    if (i + 1 < line.length() && line.substring(i, i+2).equals("*/")) {
                        in_block_comment = false;
                        i += 2;
                    } else {
                        i += 1;
                    }
                } else {
                    if (i + 1 < line.length() && line.substring(i, i+2).equals("//")) {
                        break;
                    } else if (i + 1 < line.length() && line.substring(i, i+2).equals("/*")) {
                        in_block_comment = true;
                        i += 2;
                    } else {
                        current_line_build.append(line.charAt(i));
                        i += 1;
                    }
                }
            }

            if (!in_block_comment && current_line_build.length() > 0) {
                result.add(current_line_build.toString());
            }
        }
        
        return result;
    }
}

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Problem Info

Description

Problem 722: Remove Comments - Detailed explanation and solution approaches.

Topics

Array String

Companies

Hudson River Trading

Stats

Difficulty:Medium

Languages:4

Companies:1

Problem #:722

Problem Explanation

💡

Think Before You Code

🤔 Ask Yourself

→What is the input? What output do I need?
→Can I solve a smaller version of this problem first?
→What patterns have I seen in similar problems?

✏️ Try This First

Before looking at the solution, try to solve it on paper with a simple example!

🚀 Try on LeetCode

Solution Approach

📋

Step-by-Step Solution

10 steps

1
Step 1: Initialize an empty list `result` to store the lines without comments and a boolean variable `in_block_comment` to `false`.
2
Step 2: Iterate through each line in the input `source` list.
3
Step 3: For each line, check if we are currently in a block comment. If not, reset the `current_line_build` string/StringBuilder.
4
Step 4: Iterate through each character in the current line.
5
Step 5: If `in_block_comment` is true, check for the end of the block comment (`*/`). If found, set `in_block_comment` to `false` and move the index past `*/`. Otherwise, increment the index to skip the character.
6
Step 6: If `in_block_comment` is false, check for a line comment (`//`). If found, break the inner loop, skipping the rest of the line.
7
Step 7: If `in_block_comment` is false, check for the start of a block comment (`/*`). If found, set `in_block_comment` to `true` and move the index past `/*`.
8
Step 8: If none of the above conditions are met, append the current character to `current_line_build`.
9
Step 9: After processing all characters in the line, if `in_block_comment` is false and `current_line_build` is not empty, add the line to the `result` list.
10
Step 10: Return the `result` list.

💡

Pro Tip for Students

Try tracing through each step with a small example on paper before coding. Understanding "why" each step happens is more important than memorizing the code!

⚡

Complexity Analysis

⏱️

Time Complexity

O(N)

💾

Space Complexity

O(N)

📝

Why?

The time complexity is linear because each character is processed at most a constant number of times. The space complexity is also linear because, in the worst case, the output will contain nearly all the input data.

📖New to Big O? Click to learn more

Big O Notation - Quick Guide

O(1)Best - Instant

O(log n)Excellent - Binary Search

O(n)Good - One loop

O(n log n)OK - Sorting

O(n²)Slow - Nested loops

O(2ⁿ)Very Slow - Recursion

Alternative Approaches

Regular Expressions

Using regular expressions to find and remove the comments. This could potentially be more concise but might be less efficient depending on the regex engine and the complexity of the expressions.

Time: Potentially O(N) depending on the regex engine implementation.Space: Potentially O(N) depending on the regex engine implementation.

Trade-offs: Can be more readable for those familiar with regular expressions, but might be harder to debug and potentially less efficient than the character-by-character approach.

✨

Key Insights

1Insight 1: The key is to maintain a state variable `in_block_comment` to track whether we are currently inside a block comment. This is crucial for correctly handling multi-line comments.
2Insight 2: We need to iterate through each line and each character, checking for comment delimiters (`//` and `/*`). The order of these checks is important because we want to identify comments based on their first effective occurrence.
3Insight 3: The code should handle cases where a block comment begins on one line and ends on another. The implicit newline character deletion is an important aspect addressed by maintaining the `in_block_comment` state.

📚

Key Terms Explained

For Students

Array

A collection of items stored at contiguous memory locations. Like a row of boxes.

String

A sequence of characters. Like a word or sentence in programming!

Edge Cases

Edge case 1: An empty line after removing comments. The solution correctly handles this by checking `current_line_build` is not empty before adding it to `result`.
Edge case 2: A block comment that starts and ends on the same line (e.g., `a/*comment*/b`). The solution correctly identifies and removes the comment.
Edge case 3: Nested comments. The problem statement guarantees that nested comments will not appear.
Edge case 4: A line containing both a block comment start and end `/* */`, plus regular code, like: `int x = 5; /* comment */ int y = 10;`. The solution correctly processes all parts.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Not handling the `in_block_comment` state correctly, leading to incorrect removal of code after a multi-line comment.

✗Common mistake 2: Forgetting to reset the `current_line_build` when starting a new line outside of a block comment, resulting in incorrect concatenation of lines.

✗Common mistake 3: Failing to check the boundaries of the string when looking for `//` and `/*`, causing index out of bounds errors.

✗Common mistake 4: Not handling the case where a line contains both the beginning and end of a block comment, along with actual code.

✗Common mistake 5: Memory management issues in the C implementation. Forgetting to allocate memory for the strings or free memory after use could lead to memory leaks or crashes.

✅

Learning Tip

Every mistake is a learning opportunity! When you make an error, understand why it happened before moving on.

🌍

Where is This Used in Real Life?

💼Real-world scenario 1: Code obfuscation or minification, where comments are removed to reduce the size of the code and make it harder to read.

🏦Real-world scenario 2: Static code analysis tools often need to parse code and remove comments as a preprocessing step.

🎮Real-world scenario 3: Preprocessing steps for compilers and interpreters use comment removal to clean up the source code before further processing.

💡Understanding real-world applications helps you see the "why" behind algorithms. It's not just about passing interviews—these concepts power the apps and websites you use every day!

Author's Notes

💡 Key Insight

When dealing with arrays, always consider if sorting would simplify the problem. Many array problems have O(n) solutions using hash maps or two-pointer technique.

🎯 Interview Tip

Consider multiple approaches before coding. Mentioning alternatives shows breadth of knowledge.

⚠️ Watch Out

Off-by-one errors are the most common mistake. Double-check your loop bounds and array indices.

Written by Saswata Mondal

Software Engineer • Algorithms Enthusiast • Interview Coach

👨‍💻

About the Author

👨‍💻

Saswata MondalAuthor400+ LC Problems

B.Tech CSE, IEM Kolkata | LeetCode Rating: 1672 (Top 15%)

Certified: IBM Java Developer, NPTEL Programming in Java

Last updated: June 3, 2024LeetCode Profile GitHub

This solution has been crafted based on my experience solving 400+ LeetCode problems. Each explanation is designed to help you understand the underlying concepts, not just memorize the code.

Community Tips & Insights

I was asked this exact problem at my Google interview. The interviewer wanted me to optimize space complexity.

@InterviewProJul 14

Try solving this without using extra space first. It's a great exercise for understanding the underlying pattern.

@CodeNinjaJul 7

❓

Frequently Asked Questions

What is the most efficient way to solve array problems?

Most array problems can be solved efficiently using techniques like two-pointer approach, sliding window, or hash maps. The key is identifying whether the array is sorted (enabling binary search) or if tracking seen elements (using hash maps) can help achieve O(n) time complexity.

How do I handle string manipulation efficiently?

In languages with immutable strings (Java, Python, C#), use StringBuilder/list to avoid O(n) concatenation cost. Consider using character arrays for in-place modifications. For pattern matching, consider KMP or Rabin-Karp for O(n+m) complexity.

What techniques are commonly tested in medium-difficulty problems?

Medium problems often require combining multiple techniques: two-pointer with hash maps, BFS/DFS with state tracking, dynamic programming with optimization, or divide-and-conquer approaches. They test your ability to recognize patterns and choose the right combination of techniques.

How should I practice algorithm problems effectively?

Focus on understanding patterns rather than memorizing solutions. After solving a problem, review optimal solutions and understand the intuition. Group similar problems to recognize patterns. Practice explaining your approach out loud. Review problems after a few days to test retention.

What is the optimal approach for "Remove Comments"?

For this medium-level Array problem, the key is to identify the core pattern. Analyze the input constraints to determine acceptable time complexity. Consider whether string techniques can optimize the solution. Always handle edge cases and validate your approach with examples before coding.

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