2390. Removing Stars From a String

1. Description

You are given a string s, which contains stars *.
In one operation, you can:

• Choose a star in s.
• Remove the closest non-star character to its left, as well as remove the star itself.

Return the string after all stars have been removed.

Note:

• The input will be generated such that the operation is always possible.
• It can be shown that the resulting string will always be unique.

2. Example

Example 1:
Input: s = “leet**cod*e”
Output: “lecoe”
Explanation: Performing the removals from left to right:

• The closest character to the 1st star is ’t' in “leet**cod*e”. s becomes “lee*cod*e”.
• The closest character to the 2nd star is ‘e’ in “lee*cod*e”. s becomes “lecod*e”.
• The closest character to the 3rd star is ’d' in “lecod*e”. s becomes “lecoe”.
  There are no more stars, so we return “lecoe”.

Example 2:
Input: s = “erase*****”
Output: ""
Explanation: The entire string is removed, so we return an empty string.

3. Constraints

• 1 <= s.length <= $10^5$
• s consists of lowercase English letters and stars *.
• The operation above can be performed on s.

4. Solutions

Stack

n = s.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    string removeStars(string s) {
        int i = 0;
        for (int j = 0; j < s.size(); ++j) {
            if (s[j] != '*') {
                s[i] = s[j];
                ++i;
            } else {
                --i;
            }
        }

        return s.substr(0, i);
    }
};