Process String with Special Operations I

Jun 16, 2026Madhav

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Problem Statement

You are given a string s consisting of lowercase English letters and the special characters: *, #, and %.

Build a new string result by processing s according to the following rules from left to right:

• If the letter is a lowercase English letter, append it to result.
• A * removes the last character from result, if it exists (backspace).
• A # duplicates the current result and appends it to itself.
• A % reverses the current result.

Return the final string result after processing all characters in s.

Example 1:

• Input: s = "a#b%*"
• Output: "ba"
• Explanation:
  1. 'a' -> Append: "a"
  2. '#' -> Duplicate: "aa"
  3. 'b' -> Append: "aab"
  4. '%' -> Reverse: "baa"
  5. '*' -> Remove last: "ba"

Example 2:

• Input: s = "z*#"
• Output: ""
• Explanation:
  1. 'z' -> Append: "z"
  2. '*' -> Remove last: ""
  3. '#' -> Duplicate: ""

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Approach: Simulation

Since the constraints on the length of the string are extremely small ($N \le 20$), we can solve this by directly simulating the operations using a dynamic array, list, or string builder depending on the language.

1. Maintain an empty result structure.
2. Iterate through each character of the input string s.
3. Based on the character, perform the respective operation:
   • Lowercase letter (a-z): Add to the end of the result.
   • Asterisk (*): Pop the last element (make sure to check if the result is not empty first).
   • Hash (#): Concatenate the current result to itself.
   • Percent (%): Reverse the characters in the result.
4. Once the loop finishes, return the final result casted back to a string.

Using a list or character array is typically more efficient for these operations than manipulating immutable strings directly, especially for the reverse and duplicate steps.

Complexity Analysis

• Time Complexity: $O(2^N)$ In the absolute worst-case scenario, the string size can double at each step (e.g., if the input is mostly # characters). For an input length $N$, the maximum length of the output string and the maximum number of operations required to reverse or duplicate the string would be proportional to $2^N$.
• Space Complexity: $O(2^N)$ The space required to store the intermediate and final result string can grow exponentially, up to $O(2^N)$ characters.

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Solutions

C++

#include <string>
#include <algorithm>

using namespace std;

class Solution {
public:
    string processStr(string s) {
        string ans = "";
        for (char c : s) {
            if (c >= 'a' && c <= 'z') {
                ans.push_back(c);
            } else if (c == '*') {
                if (!ans.empty()) {
                    ans.pop_back();
                }
            } else if (c == '#') {
                ans += ans;
            } else if (c == '%') {
                reverse(ans.begin(), ans.end());
            }
        }
        return ans;
    }
};

Java

class Solution {
    public String processStr(String s) {
        StringBuilder ans = new StringBuilder();
        for (char c : s.toCharArray()) {
            if (c >= 'a' && c <= 'z') {
                ans.append(c);
            } else if (c == '*') {
                if (ans.length() > 0) {
                    ans.deleteCharAt(ans.length() - 1);
                }
            } else if (c == '#') {
                ans.append(ans);
            } else if (c == '%') {
                ans.reverse();
            }
        }
        return ans.toString();
    }
}

Python

class Solution:
    def processStr(self, s: str) -> str:
        ans = []
        for c in s:
            if 'a' <= c <= 'z':
                ans.append(c)
            elif c == '*':
                if ans:
                    ans.pop()
            elif c == '#':
                ans.extend(ans)
            elif c == '%':
                ans.reverse()
        return "".join(ans)

JavaScript

/**
 * @param {string} s
 * @return {string}
 */
var processStr = function(s) {
    let ans = [];
    for (let c of s) {
        if (c >= 'a' && c <= 'z') {
            ans.push(c);
        } else if (c === '*') {
            if (ans.length > 0) {
                ans.pop();
            }
        } else if (c === '#') {
            ans = ans.concat(ans);
        } else if (c === '%') {
            ans.reverse();
        }
    }
    return ans.join("");
};