2390. Removing Stars From a String

Problem

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.

  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.

  Constraints:

• 1 <= s.length <= 10^5

• s consists of lowercase English letters and stars *.

• The operation above can be performed on s.

Solution (Java)

class Solution {
    public String removeStars(String s) {
        StringBuilder sb = new StringBuilder();
        int stars = 0;
        for (int i = s.length() - 1; i >= 0; --i) {
            if (s.charAt(i) == '*') {
                ++stars;
            } else if (stars > 0) {
                --stars;
            } else {
                sb.append(s.charAt(i));
            }
        }
        return sb.reverse().toString();
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).