Watch 7 video solutions for Remove Comments, a medium level problem involving Array, String. This walkthrough by Aryan Codes has 3,454 views views. Want to try solving it yourself? Practice on FleetCode or read the detailed text solution.
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.
| 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. |