722. Remove Comments


Given a C++ program, remove comments from it. The program source is an array 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 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: 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;", "}"]

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;
}

Output: ["int main()","{ ","  ","int a, b, c;","a = b + c;","}"]

The line by line code is visualized as below:
int main()
{ 
  
int a, b, c;
a = b + c;
}

Explanation: 
The string /* denotes a block comment, including line 1 and lines 6-9. The string // denotes line 4 as comments.

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"].

Note:

  • The length of source is in the range [1, 100].
  • The length of source[i] is in the range [0, 80].
  • Every open block comment is eventually closed.
  • There are no single-quote, double-quote, or control characters in the source code.


    • Approach #1: Parsing [Accepted]

    Intuition and Algorithm

    We need to parse the source line by line. Our state is that we either are in a block comment or not.

    • If we start a block comment and we aren't in a block, then we will skip over the next two characters and change our state to be in a block.

    • If we end a block comment and we are in a block, then we will skip over the next two characters and change our state to be not in a block.

    • If we start a line comment and we aren't in a block, then we will ignore the rest of the line.

    • If we aren't in a block comment (and it wasn't the start of a comment), we will record the character we are at.

    • At the end of each line, if we aren't in a block, we will record the line.

    Python

    class Solution(object):
    def removeComments(self, source):
        in_block = False
        ans = []
        for line in source:
            i = 0
            if not in_block:
                newline = []
            while i < len(line):
                if line[i:i+2] == '/*' and not in_block:
                    in_block = True
                    i += 1
                elif line[i:i+2] == '*/' and in_block:
                    in_block = False
                    i += 1
                elif not in_block and line[i:i+2] == '//':
                    break
                elif not in_block:
                    newline.append(line[i])
                i += 1
            if newline and not in_block:
                ans.append("".join(newline))

        return ans

    Java

    class Solution {
    public List<String> removeComments(String[] source) {
        boolean inBlock = false;
        StringBuilder newline = new StringBuilder();
        List<String> ans = new ArrayList();
        for (String line: source) {
            int i = 0;
            char[] chars = line.toCharArray();
            if (!inBlock) newline = new StringBuilder();
            while (i < line.length()) {
                if (!inBlock && i+1 < line.length() && chars[i] == '/' && chars[i+1] == '*') {
                    inBlock = true;
                    i++;
                } else if (inBlock && i+1 < line.length() && chars[i] == '*' && chars[i+1] == '/') {
                    inBlock = false;
                    i++;
                } else if (!inBlock && i+1 < line.length() && chars[i] == '/' && chars[i+1] == '/') {
                    break;
                } else if (!inBlock) {
                    newline.append(chars[i]);
                }
                i++;
            }
            if (!inBlock && newline.length() > 0) {
                ans.add(new String(newline));
            }
        }
        return ans;
    }
}

    Complexity Analysis

    • Time Complexity: , where is the total length of the source code.

    • Space Complexity: , the space used by recording the source code into ans.

    Analysis written by: @awice.

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