#3324

Medium

LeetCode Problem Workspace

Find the Sequence of Strings Appeared on the Screen

Simulate Alice typing a target string with a special keyboard that appends letters one by one. Solve using string simulation techniques.

string simulation

Problem Statement

You are given a string target. Alice is going to type this target on her computer using a special keyboard that has only two keys: key 1 appends the letter 'a' and key 2 appends the current string itself. Initially, there is an empty string "" on the screen, so she can only press key 1.

Your task is to return the sequence of strings that appear on the screen after each key press, including the final target string.

Examples

Example 1

Input: target = "abc"

Output: ["a","aa","ab","aba","abb","abc"]

The sequence of key presses done by Alice are:

Example 2

Input: target = "he"

Output: ["a","b","c","d","e","f","g","h","ha","hb","hc","hd","he"]

Example details omitted.

Constraints

1 <= target.length <= 400
target consists only of lowercase English letters.

Solution Approach

Simulate Key Presses

To build the sequence, iterate through each character of the target string. Start with the empty string and use key presses to append 'a' or the current string, following the rules of the problem.

Track Each Intermediate Step

After each key press, record the string on the screen. Track this step-by-step until you match the target string.

Optimize the Simulation

Consider optimizing the simulation by using dynamic programming or caching results to avoid redundant calculations when repeating sequences.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time and space complexity depend on the chosen approach. A brute-force solution may have high time complexity due to string operations, but optimization techniques can help reduce it significantly.

What Interviewers Usually Probe

The candidate understands string manipulation and simulation patterns.
The candidate demonstrates an ability to break down problems into small, manageable steps.
The candidate shows good understanding of how to optimize repeated operations.

Common Pitfalls or Variants

Common pitfalls

Forgetting to track intermediate steps in the sequence.
Not handling the case where key 2 appends the current string properly.
Using inefficient string operations that lead to poor performance.

Follow-up variants

Optimize the simulation using dynamic programming to improve time complexity.
Consider varying the keyboard functionality, e.g., introducing more keys.
Extend the problem to support strings with different character sets.

FAQ

What is the main approach to solving the problem "Find the Sequence of Strings Appeared on the Screen"?

The main approach is simulating the typing process by considering how key presses build the sequence, focusing on appending the correct strings.

How do I handle the string appending in the problem?

You should carefully track the intermediate strings that form the sequence as Alice presses keys. Consider using efficient string handling techniques to avoid unnecessary repetition.

What are the key constraints for this problem?

The target string length is between 1 and 400, and it only consists of lowercase English letters.

How can I optimize the solution to avoid redundant calculations?

You can optimize by using dynamic programming or caching previously calculated strings to avoid recalculating them multiple times.

What is the time complexity of the brute-force approach?

The time complexity can be high with a brute-force approach due to repeated string operations, but optimizations can reduce this significantly.

terminal

Solution

Solution 1: Simulation

We can simulate Alice's typing process, starting from an empty string and updating the string after each keystroke until the target string is obtained.

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class Solution:
    def stringSequence(self, target: str) -> List[str]:
        ans = []
        for c in target:
            s = ans[-1] if ans else ""
            for a in ascii_lowercase:
                t = s + a
                ans.append(t)
                if a == c:
                    break
        return ans

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