Process String with Special Operations II

To solve this problem, simulate the operations on the string left to right, keeping track of the changes in length. Ensure that the final result is calculated correctly based on the value of k, considering bounds and operation effects on the string.

Problem Statement

You are given a string s consisting of lowercase English letters and the special characters: '*', '#', and '%'. You are also given an integer k. Build a new string result by processing s according to the following rules from left to right:

1. The '*' character doubles the last character of the current result. 2. The '#' character removes the last character of the current result. 3. The '%' character reverses the string result so far. After processing all operations, return the character at index k of the final result. If k is out of bounds, return a dot ('.').

Examples

Example 1

Input: s = "a#b%*", k = 1

Output: "a"

The final result is "ba" . The character at index k = 1 is 'a' .

Example 2

Input: s = "cd%#*#", k = 3

Output: "d"

The final result is "dcddcd" . The character at index k = 3 is 'd' .

Example 3

Input: s = "z*#", k = 0

Output: "."

The final result is "" . Since index k = 0 is out of bounds, the output is '.' .

Constraints

• 1 <= s.length <= 105
• s consists of only lowercase English letters and special characters '*', '#', and '%'.
• 0 <= k <= 1015
• The length of result after processing s will not exceed 1015.

Solution Approach

Simulate Operations

Loop through the string, applying the appropriate operation on the result string based on the encountered special characters. Adjust the length of the result string dynamically to match the current operations.

Track Result Length

As operations can alter the length of the result string (especially '*' and '#'), track the changes to avoid excessive memory usage, particularly when processing large inputs.

Handle Large k

Ensure that the value of k is within bounds of the final string length after all operations. If k exceeds the string length, return a dot '.' as the result.

Complexity Analysis

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

Time complexity depends on the approach, as each operation (except for reverse) can modify the result string, and tracking the string length efficiently is crucial for large inputs. Space complexity is determined by the storage needed for the result string, which could grow large depending on the operations.

What Interviewers Usually Probe

• Assess the candidate's ability to simulate string manipulations and track result length.
• Look for efficient handling of the k-bound condition, especially with large k values.
• Check the candidate's awareness of performance trade-offs when dealing with operations that modify string length or order.

Common Pitfalls or Variants

Common pitfalls

• Failing to correctly handle the reverse '%' operation, especially when tracking string length.
• Mismanaging the string size, leading to memory inefficiency when dealing with large strings.
• Overlooking cases where k is out of bounds, causing incorrect results.

Follow-up variants

• Introduce a constraint where k is much larger than the string's length, forcing more efficient bounds checking.
• Modify the problem to include more operations, such as appending a character or removing a specific character.
• Change the problem to return the entire processed string instead of just one character.

FAQ

What is the pattern used in "Process String with Special Operations II"?

The problem follows the string simulation pattern, where operations like '*', '#', and '%' modify the string based on specific rules.

How should I handle large k values in this problem?

Ensure that you track the length of the string carefully to handle large k values. If k is out of bounds after processing the string, return a dot ('.').

What happens when a reverse operation '%' is encountered?

The '%' operation reverses the string built so far, which can significantly change the order of characters. Keep track of the result length before and after reversing.

How do I efficiently manage the result string size?

You need to carefully manage the string size by directly applying operations without unnecessarily expanding or contracting the string too frequently, as this can lead to inefficiency.

What should I do if the string is very large in this problem?

When dealing with large strings, focus on efficiently simulating operations and tracking the string's state without creating unnecessary copies or excessive memory usage.