3612. Process String with Special Operations I

Time: O(n^2)

Space: O(n)

Problem Overview

Process the string left to right and maintain the current result as a mutable sequence.

Intuition

Process the string left to right and maintain the current result as a mutable sequence. Letters append. The * character removes the last character if any exist. The # character duplicates the current result by appending it to itself. The % character reverses the entire current result in place. After all characters, convert the mutable result to a string.

Algorithm

1Create an empty mutable result (StringBuilder).
2Scan characters from left to right.
3If the character is a lowercase English letter, append it to result.
4If the character is the * operator and result is not empty, remove the last character.
5If the character is the # operator, append the current result to itself to duplicate it.
6If the character is the % operator, reverse the entire current result in place.
7Return result as a string.

Example Walkthrough

Input: s = "a#b%*"

1.Start result = "".
2.Read a → append → "a".
3.Read # → duplicate → "aa".
4.Read b → append → "aab".
5.Read % → reverse → "baa".
6.Read * → remove last → "ba".

Output: "ba"

Common Pitfalls

•If * appears when result is empty, do nothing.
•For #, duplicate the current content by appending a copy of the builder, not only one character.
•For %, reverse the whole current result, not only the next segment.

3612.cs

// Approach: Simulate the operations on a mutable result string.
// Keep a StringBuilder for the current result.
// For each character in s:
// - If it is a lowercase letter, append it.
// - If it is '*', delete the last character of the current result (if it exists).
// - If it is '#', duplicate the current result by appending it to itself.
// - If it is '%', reverse the current result in place.
// Time: O(n^2) Space: O(n)
public class Solution
{
    public string ProcessStr(string s)
    {
        StringBuilder result = new StringBuilder();

        foreach (char c in s)
        {
            if (char.IsLetter(c))
                result.Append(c);
            else if (c == '*')
            {
                if (result.Length > 0)
                    result.Length = result.Length - 1;
            }
            else if (c == '#')
                result.Append(result.ToString());
            else if (c == '%')
            {
                // Reverse the current result string
                int len = result.Length;
                for (int i = 0; i < len / 2; i++)
                {
                    char temp = result[i];
                    result[i] = result[len - 1 - i];
                    result[len - 1 - i] = temp;
                }
            }
            // Other characters are ignored
        }

        return result.ToString();
    }
}

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