Process String with Special Operations I - Solution & Explanation

Problem Overview: You receive a string that contains normal characters mixed with special operation characters. Each operation modifies the current result string (for example deleting, reversing, or altering previously processed characters). The task is to process the string from left to right and return the final string after applying every operation.

Approach 1: Naive String Simulation (O(n²) time, O(n) space)

The most direct approach is to simulate the process using a mutable string and apply operations immediately as they appear. Iterate through the input and append regular characters to the result string. When a special operation appears, modify the string accordingly (for example removing the last character or reversing the entire string). Operations like full reversal or repeated string concatenation can cost O(n) each, which pushes the worst‑case complexity to O(n²). This approach is simple and mirrors the problem statement, but it becomes slow when the string grows large.

Approach 2: Optimized Simulation with Stack / String Builder (O(n) time, O(n) space)

A more efficient strategy uses a dynamic container such as a stack, list, or StringBuilder. Iterate through the input exactly once. Regular characters are appended to the container. When an operation appears, apply the corresponding modification directly to the container (for example pop for deletion operations or toggling a flag for operations like reversal). By avoiding repeated full-string rebuilds, every character is processed at most once, giving O(n) time complexity and O(n) auxiliary space.

Many implementations also use a boolean flag to represent logical reversal instead of physically reversing the string each time. Characters are appended to either the front or back depending on the flag, typically using a deque. This technique keeps each operation constant time while preserving the correct order of characters.

This problem is primarily a simulation exercise combined with careful string manipulation from the string category. The key idea is to represent the evolving result efficiently and apply operations without repeatedly rebuilding the entire string.

Recommended for interviews: The optimized simulation using a stack, deque, or string builder. Interviewers expect you to process the string in one pass and avoid expensive operations like repeated reversal or concatenation. Showing the naive simulation demonstrates understanding, but the O(n) implementation proves you can reason about performance and data structure choices.