722. Remove Comments

Description

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 "/*" 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.

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.

Solutions

Solution 1: Case Analysis

We use a variable \(\textit{blockComment}\) to indicate whether we are currently in a block comment. Initially, \(\textit{blockComment}\) 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 \(\textit{blockComment}\) 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 \(\textit{blockComment}\) 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 \(\textit{blockComment}\) 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.

Python3JavaC++GoTypeScriptRust

class Solution:
    def removeComments(self, source: List[str]) -> List[str]:
        ans = []
        t = []
        block_comment = False
        for s in source:
            i, m = 0, len(s)
            while i < m:
                if block_comment:
                    if i + 1 < m and s[i : i + 2] == "*/":
                        block_comment = False
                        i += 1
                else:
                    if i + 1 < m and s[i : i + 2] == "/*":
                        block_comment = True
                        i += 1
                    elif i + 1 < m and s[i : i + 2] == "//":
                        break
                    else:
                        t.append(s[i])
                i += 1
            if not block_comment and t:
                ans.append("".join(t))
                t.clear()
        return ans

class Solution {
    public List<String> removeComments(String[] source) {
        List<String> ans = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        boolean blockComment = false;
        for (String s : source) {
            int m = s.length();
            for (int i = 0; i < m; ++i) {
                if (blockComment) {
                    if (i + 1 < m && s.charAt(i) == '*' && s.charAt(i + 1) == '/') {
                        blockComment = false;
                        ++i;
                    }
                } else {
                    if (i + 1 < m && s.charAt(i) == '/' && s.charAt(i + 1) == '*') {
                        blockComment = true;
                        ++i;
                    } else if (i + 1 < m && s.charAt(i) == '/' && s.charAt(i + 1) == '/') {
                        break;
                    } else {
                        sb.append(s.charAt(i));
                    }
                }
            }
            if (!blockComment && sb.length() > 0) {
                ans.add(sb.toString());
                sb.setLength(0);
            }
        }
        return ans;
    }
}

class Solution {
public:
    vector<string> removeComments(vector<string>& source) {
        vector<string> ans;
        string t;
        bool blockComment = false;
        for (auto& s : source) {
            int m = s.size();
            for (int i = 0; i < m; ++i) {
                if (blockComment) {
                    if (i + 1 < m && s[i] == '*' && s[i + 1] == '/') {
                        blockComment = false;
                        ++i;
                    }
                } else {
                    if (i + 1 < m && s[i] == '/' && s[i + 1] == '*') {
                        blockComment = true;
                        ++i;
                    } else if (i + 1 < m && s[i] == '/' && s[i + 1] == '/') {
                        break;
                    } else {
                        t.push_back(s[i]);
                    }
                }
            }
            if (!blockComment && !t.empty()) {
                ans.emplace_back(t);
                t.clear();
            }
        }
        return ans;
    }
};

func removeComments(source []string) (ans []string) {
    t := []byte{}
    blockComment := false
    for _, s := range source {
        m := len(s)
        for i := 0; i < m; i++ {
            if blockComment {
                if i+1 < m && s[i] == '*' && s[i+1] == '/' {
                    blockComment = false
                    i++
                }
            } else {
                if i+1 < m && s[i] == '/' && s[i+1] == '*' {
                    blockComment = true
                    i++
                } else if i+1 < m && s[i] == '/' && s[i+1] == '/' {
                    break
                } else {
                    t = append(t, s[i])
                }
            }
        }
        if !blockComment && len(t) > 0 {
            ans = append(ans, string(t))
            t = []byte{}
        }
    }
    return
}

function removeComments(source: string[]): string[] {
    const ans: string[] = [];
    const t: string[] = [];
    let blockComment = false;
    for (const s of source) {
        const m = s.length;
        for (let i = 0; i < m; ++i) {
            if (blockComment) {
                if (i + 1 < m && s.slice(i, i + 2) === '*/') {
                    blockComment = false;
                    ++i;
                }
            } else {
                if (i + 1 < m && s.slice(i, i + 2) === '/*') {
                    blockComment = true;
                    ++i;
                } else if (i + 1 < m && s.slice(i, i + 2) === '//') {
                    break;
                } else {
                    t.push(s[i]);
                }
            }
        }
        if (!blockComment && t.length) {
            ans.push(t.join(''));
            t.length = 0;
        }
    }
    return ans;
}

impl Solution {
    pub fn remove_comments(source: Vec<String>) -> Vec<String> {
        let mut ans: Vec<String> = Vec::new();
        let mut t: Vec<String> = Vec::new();
        let mut blockComment = false;

        for s in &source {
            let m = s.len();
            let mut i = 0;
            while i < m {
                if blockComment {
                    if i + 1 < m && &s[i..i + 2] == "*/" {
                        blockComment = false;
                        i += 2;
                    } else {
                        i += 1;
                    }
                } else {
                    if i + 1 < m && &s[i..i + 2] == "/*" {
                        blockComment = true;
                        i += 2;
                    } else if i + 1 < m && &s[i..i + 2] == "//" {
                        break;
                    } else {
                        t.push(s.chars().nth(i).unwrap().to_string());
                        i += 1;
                    }
                }
            }
            if !blockComment && !t.is_empty() {
                ans.push(t.join(""));
                t.clear();
            }
        }
        ans
    }
}