LeetCode 3612: Process String with Special Operations I

Meta Data

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Difficulty: medium First Attempt: 2025-07-13

• Submissions: Wrong Answer → Accepted

Approach

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Simulate the rules (String Template 1). With a list as the result, * pops, # is res += res, % reverses, a letter appends, and join at the end.

My Attempts

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Attempt 1 wrote if char == "*" and res:. With an empty result that condition fails, so * falls through to the last elif char.lower() == char, and since "*".lower() is still "*", it was appended as if it were a letter. For "*" the answer is the empty string; it returned "*". Moving if res inside the branch passed.

Solution

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class Solution:
    def processStr(self, s: str) -> str:
        res = []
        for ch in s:
            if ch == "*":
                if res:
                    res.pop()
            elif ch == "#":
                res += res
            elif ch == "%":
                res.reverse()
            else:
                res.append(ch)
        return "".join(res)

Complexity

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The result can double at every #; time and space are proportional to the final length.

Mistakes to Avoid

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• Folding an extra condition into a branch test with and makes failures fall through to later branches.
• ch.lower() == ch does not detect letters; symbols pass too. Use ch.isalpha() or just else.

Interview Explanation

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I simulate the four operations on a list and join at the end. Note that * on an empty result does nothing; that check must sit inside the branch, or * falls through and is treated as a letter. Time is proportional to the output length.