Given a C++ program, remove comments from it. The program source is an array of strings source where source[i] is the ith 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.
"//" 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."/*" denotes a block comment, which represents that all characters until the next (non-overlapping) occurrence of "*/" should be ignored. (Here, occurrences happen in reading order: line by line from left to right.) To be clear, the string "/*/" does not yet end the block comment, as the ending would be overlapping the beginning.The first effective comment takes precedence over others.
"//" occurs in a block comment, it is ignored."/*" occurs in a line or block comment, it is also ignored.If a certain line of code is empty after removing comments, you must not output that line: each string in the answer list will be non-empty.
There will be no control characters, single quote, or double quote characters.
source = "string s = "/* Not a comment. */";" will not be a test case.Also, nothing else such as defines or macros will interfere with the comments.
It is guaranteed that every open block comment will eventually be closed, so "/*" outside of a line or block comment always starts a new comment.
Finally, implicit newline characters can be deleted by block comments. Please see the examples below for details.
After removing the comments from the source code, return the source code in the same format.
Example 1:
Input: source = ["/*Test program */", "int main()", "{ ", " // variable declaration ", "int a, b, c;", "/* This is a test", " multiline ", " comment for ", " testing */", "a = b + c;", "}"]
Output: ["int main()","{ "," ","int a, b, c;","a = b + c;","}"]
Explanation: The line by line code is visualized as below:
/*Test program */
int main()
{
// variable declaration
int a, b, c;
/* This is a test
multiline
comment for
testing */
a = b + c;
}
The string /* denotes a block comment, including line 1 and lines 6-9. The string // denotes line 4 as comments.
The line by line output code is visualized as below:
int main()
{
int a, b, c;
a = b + c;
}
Example 2:
Input: source = ["a/*comment", "line", "more_comment*/b"] Output: ["ab"] Explanation: The original source string is "a/*comment\nline\nmore_comment*/b", where we have bolded the newline characters. After deletion, the implicit newline characters are deleted, leaving the string "ab", which when delimited by newline characters becomes ["ab"].
Constraints:
1 <= source.length <= 1000 <= source[i].length <= 80source[i] consists of printable ASCII characters.Problem Overview: You receive source code as an array of strings. The task is to remove both line comments (//) and block comments (/* ... */) and return the cleaned code. Block comments may span multiple lines, while line comments terminate the rest of the current line.
The challenge is correctly identifying comment boundaries while preserving all valid characters outside comment regions. The parser must track whether it is currently inside a block comment and handle transitions when encountering comment markers.
Approach 1: Single Pass Parsing (O(n) time, O(n) space)
This approach scans the entire source code once while maintaining a boolean flag inBlock to indicate whether the parser is currently inside a block comment. Iterate through each line and examine characters sequentially. When // appears and you're not inside a block comment, ignore the rest of that line. When /* appears, switch inBlock to true and skip characters until */ is found. Characters encountered while inBlock is false are appended to a buffer that forms the cleaned line.
When a line finishes and you're not inside a block comment, push the buffer to the result list and reset it. The key insight is treating the entire input as a continuous stream while preserving line boundaries only when appropriate. This technique works well for problems involving string parsing and array traversal. Time complexity is O(n), where n is the total number of characters across all lines, and space complexity is O(n) for the output.
Approach 2: Two-Pointer Iterative Parsing (O(n) time, O(n) space)
This version explicitly uses two indices while scanning each line: one pointer reads characters and the other effectively marks valid output positions in a buffer. The algorithm checks pairs of characters (i and i+1) to detect comment tokens like //, /*, and */. When inside a block comment, the read pointer advances until the closing token is found.
The advantage of the two-pointer approach is clarity in state transitions. It makes comment detection explicit and avoids repeatedly slicing strings. Each character is processed at most once, producing the same O(n) time complexity with O(n) extra space for the resulting cleaned lines. This technique is common in interview problems involving streaming text or incremental string processing.
Recommended for interviews: The single-pass parsing approach is typically expected. It demonstrates clean state management and efficient linear scanning. Interviewers often look for correct handling of multi-line block comments and edge cases such as comment markers appearing inside existing block comments. Showing a brute-force mindset first (thinking about scanning and detecting markers) helps demonstrate reasoning, but implementing the single-pass state machine shows strong problem-solving skills.
This approach involves iterating through each line of the source code while managing a flag to detect if we are currently in a block comment. As we parse each line, we handle three different states: whether we are inside a block comment, encountering a line comment, or processing regular code.
This code iterates over each line of the source code. It checks for the start '/*', end '*/' of block comments, and line comments '//'. It uses a flag in_block to monitor if current parsing is done inside a block comment. Characters are added to a temporary string newline unless part of a comment. Once parsing completes, it captures all the cleaned up code lines in the result list which it returns.
Time Complexity: O(n*m), where n is the number of lines and m is the average length of each line. Space Complexity: O(n*m), for the resulting list storage.
In this approach, two pointers are used. A pointer iterates through the string to seek comment delimiters ('//', '/*', '*/') while another manages the indices of the non-commented part of each line. It is effective in keeping track of multi-line parsing for block comments.
This Java solution uses two-pointers to iterate through the input lines. A StringBuilder is used to construct non-commented sections of the code as we iterate each line. By utilizing subnet comparisons, the approach efficiently detects comment starter and end patterns and processes the valid source code accordingly.
Java
JavaScript
Time Complexity: O(n*m), where n is the number of lines and m is the average length of each line. Space Complexity: O(n*m), needed for result list and string manipulations.
We use a variable blockComment to indicate whether we are currently in a block comment. Initially, blockComment is false. We use a variable t to store the valid characters of the current line.
Next, we traverse each line and discuss the following cases:
If we are currently in a block comment, and the current character and the next character are '*/', it means the block comment ends. We set blockComment to false and skip these two characters. Otherwise, we continue in the block comment state without doing anything.
If we are not currently in a block comment, and the current character and the next character are '/*', it means a block comment starts. We set blockComment to true and skip these two characters. If the current character and the next character are '//', it means a line comment starts, and we exit the current line traversal. Otherwise, the current character is a valid character, and we add it to t.
After traversing the current line, if blockComment is false and t is not empty, it means the current line is valid. We add it to the answer array and clear t. Continue to traverse the next line.
The time complexity is O(L), and the space complexity is O(L), where L is the total length of the source code.
| Approach | Complexity |
|---|---|
| Single Pass Parsing | Time Complexity: O(n*m), where n is the number of lines and m is the average length of each line. Space Complexity: O(n*m), for the resulting list storage. |
| Two-Pointer Iterative Parsing | Time Complexity: O(n*m), where n is the number of lines and m is the average length of each line. Space Complexity: O(n*m), needed for result list and string manipulations. |
| Case Analysis | — |
| Approach | Time | Space | When to Use |
|---|---|---|---|
| Single Pass Parsing | O(n) | O(n) | Best general solution. Efficient when scanning the entire code stream once. |
| Two-Pointer Iterative Parsing | O(n) | O(n) | Useful when implementing explicit character scanning logic or avoiding repeated substring operations. |
Remove Comments | Leetcode • Aryan Codes • 3,454 views views
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