Process String with Special Operations I

Process String with Special Operations I

What is this problem about?

The Process String with Special Operations I problem gives you a string with special characters: '*' removes the previous character, and '#' reverses the current string so far. Process all operations and return the final string. This coding problem uses a stack or deque with operation simulation. The string and simulation interview pattern is demonstrated.

Why is this asked in interviews?

Amazon asks this to test string simulation with special operations — a pattern seen in text editors (backspace handling), command history, and undo systems. The challenge is efficiently handling both deletion and reversal operations.

Algorithmic pattern used

Stack or deque simulation. Process characters left to right: if regular char, append to result. If '*', pop the last character (if not empty). If '#', reverse the current result. Return the final string.

Example explanation

s="abc#def*".

• 'a': result="a".
• 'b': result="ab".
• 'c': result="abc".
• '#': result="cba" (reverse).
• 'd': result="cbad".
• 'e': result="cbade".
• 'f': result="cbadef".
• '*': result="cbade" (remove 'f'). Final: "cbade".

Common mistakes candidates make

• Applying '*' before checking if result is empty.
• '#' reversing the wrong portion.
• Not handling consecutive operations correctly.
• Using a list and converting to string each step (O(n²) — prefer deque or join at end).

Interview preparation tip

String simulation problems with special commands (delete, reverse, rotate) map naturally to deque/stack operations. Always process operations left to right, maintaining the current state. For efficiency: build incrementally and apply reversals lazily if possible. Practice similar problems: "string with backspace," "build array with operations," "text editor simulation." These are common in systems programming and IDE implementation interviews.