Remove Comments

Problem

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.

• 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 "/*" 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.

• For example, if the string "//" occurs in a block comment, it is ignored.
• Similarly, if the string "/*" 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.

• For example, 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.

Examples

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 <= 100
• 0 <= source[i].length <= 80
• source[i] consists of printable ASCII characters.
• Every open block comment is eventually closed.
• There are no single-quote or double-quote in the input.

Solution

Method 1 - Simulate Parsing

Intuition

We need to simulate parsing the code, tracking whether we are inside a block comment or not, and building the output line by line, skipping commented parts.

Approach

Iterate through each line and each character, maintaining a flag for block comments. If we see // outside a block, ignore the rest of the line. If we see /*, enter block mode and ignore until */ is found. Only add non-commented code to the output.

Code

C++

#include <vector>
#include <string>
using namespace std;
class Solution {
public:
    vector<string> removeComments(vector<string>& source) {
        vector<string> res;
        bool block = false;
        string newline;
        for (auto& line : source) {
            int i = 0, n = line.size();
            if (!block) newline = "";
            while (i < n) {
                if (!block && i + 1 < n && line[i] == '/' && line[i+1] == '/') break;
                else if (!block && i + 1 < n && line[i] == '/' && line[i+1] == '*') {
                    block = true; i += 2;
                } else if (block && i + 1 < n && line[i] == '*' && line[i+1] == '/') {
                    block = false; i += 2;
                } else if (!block) {
                    newline += line[i++];
                } else {
                    i++;
                }
            }
            if (!block && !newline.empty()) res.push_back(newline);
        }
        return res;
    }
};

Go

func removeComments(source []string) []string {
    res := []string{}
    block := false
    newline := ""
    for _, line := range source {
        i, n := 0, len(line)
        if !block {
            newline = ""
        }
        for i < n {
            if !block && i+1 < n && line[i] == '/' && line[i+1] == '/' {
                break
            } else if !block && i+1 < n && line[i] == '/' && line[i+1] == '*' {
                block = true; i += 2
            } else if block && i+1 < n && line[i] == '*' && line[i+1] == '/' {
                block = false; i += 2
            } else if !block {
                newline += string(line[i]); i++
            } else {
                i++
            }
        }
        if !block && len(newline) > 0 {
            res = append(res, newline)
        }
    }
    return res
}

Java

class Solution {
    public List<String> removeComments(String[] source) {
        List<String> res = new ArrayList<>();
        boolean block = false;
        StringBuilder newline = new StringBuilder();
        for (String line : source) {
            int i = 0, n = line.length();
            if (!block) newline = new StringBuilder();
            while (i < n) {
                if (!block && i + 1 < n && line.charAt(i) == '/' && line.charAt(i+1) == '/') break;
                else if (!block && i + 1 < n && line.charAt(i) == '/' && line.charAt(i+1) == '*') {
                    block = true; i += 2;
                } else if (block && i + 1 < n && line.charAt(i) == '*' && line.charAt(i+1) == '/') {
                    block = false; i += 2;
                } else if (!block) {
                    newline.append(line.charAt(i++));
                } else {
                    i++;
                }
            }
            if (!block && newline.length() > 0) res.add(newline.toString());
        }
        return res;
    }
}

Kotlin

class Solution {
    fun removeComments(source: Array<String>): List<String> {
        val res = mutableListOf<String>()
        var block = false
        var newline = StringBuilder()
        for (line in source) {
            var i = 0
            val n = line.length
            if (!block) newline = StringBuilder()
            while (i < n) {
                if (!block && i + 1 < n && line[i] == '/' && line[i+1] == '/') break
                else if (!block && i + 1 < n && line[i] == '/' && line[i+1] == '*') {
                    block = true; i += 2
                } else if (block && i + 1 < n && line[i] == '*' && line[i+1] == '/') {
                    block = false; i += 2
                } else if (!block) {
                    newline.append(line[i]); i++
                } else {
                    i++
                }
            }
            if (!block && newline.isNotEmpty()) res.add(newline.toString())
        }
        return res
    }
}

Python

from typing import List
class Solution:
    def removeComments(self, source: List[str]) -> List[str]:
        res = []
        block = False
        newline = ''
        for line in source:
            i, n = 0, len(line)
            if not block:
                newline = ''
            while i < n:
                if not block and i+1 < n and line[i:i+2] == '//':
                    break
                elif not block and i+1 < n and line[i:i+2] == '/*':
                    block = True
                    i += 2
                elif block and i+1 < n and line[i:i+2] == '*/':
                    block = False
                    i += 2
                elif not block:
                    newline += line[i]
                    i += 1
                else:
                    i += 1
            if not block and newline:
                res.append(newline)
        return res

Rust

impl Solution {
    pub fn remove_comments(source: Vec<String>) -> Vec<String> {
        let mut res = Vec::new();
        let mut block = false;
        let mut newline = String::new();
        for line in source.iter() {
            let mut i = 0;
            let n = line.len();
            let chars: Vec<char> = line.chars().collect();
            if !block {
                newline = String::new();
            }
            while i < n {
                if !block && i+1 < n && chars[i] == '/' && chars[i+1] == '/' {
                    break;
                } else if !block && i+1 < n && chars[i] == '/' && chars[i+1] == '*' {
                    block = true; i += 2;
                } else if block && i+1 < n && chars[i] == '*' && chars[i+1] == '/' {
                    block = false; i += 2;
                } else if !block {
                    newline.push(chars[i]); i += 1;
                } else {
                    i += 1;
                }
            }
            if !block && !newline.is_empty() {
                res.push(newline.clone());
            }
        }
        res
    }
}

TypeScript

function removeComments(source: string[]): string[] {
    const res: string[] = [];
    let block = false;
    let newline = '';
    for (const line of source) {
        let i = 0, n = line.length;
        if (!block) newline = '';
        while (i < n) {
            if (!block && i+1 < n && line[i] === '/' && line[i+1] === '/') break;
            else if (!block && i+1 < n && line[i] === '/' && line[i+1] === '*') {
                block = true; i += 2;
            } else if (block && i+1 < n && line[i] === '*' && line[i+1] === '/') {
                block = false; i += 2;
            } else if (!block) {
                newline += line[i]; i++;
            } else {
                i++;
            }
        }
        if (!block && newline) res.push(newline);
    }
    return res;
}

Complexity

• ⏰ Time complexity: O(N*L) where N is the number of lines and L is the average line length.
• 🧺 Space complexity: O(N*L) for the result.