In The Stars

Unit 7 Session 1 (Click for link to problem statements)

Problem Highlights

• 💡 Difficulty: Easy
• ⏰ Time to complete: 10 mins
• 🛠️ Topics: Recursion, String Manipulation, Iteration

1: U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

• Established a set (2-3) of test cases to verify their own solution later.
• Established a set (1-2) of edge cases to verify their solution handles complexities.
• Have fully understood the problem and have no clarifying questions.
• Have you verified any Time/Space Constraints for this problem?

• Q: What should happen if the string is empty or has only one character?
  • A: If the string is empty or has one character, it should be returned as is without any stars.

HAPPY CASE
Input: "hello"
Output: "h*e*l*l*o"
Explanation: Stars are inserted between each character.

EDGE CASE
Input: "h"
Output: "h"
Explanation: No stars are added as there's only one character.

2: M-atch

Match what this problem looks like to known categories of problems, e.g. Linked List or Dynamic Programming, and strategies or patterns in those categories.

This problem is a string manipulation challenge that can be solved using both recursion and iteration:

• Using recursion to break down the problem and insert stars between each consecutive character.
• Using iteration to traverse through the string and construct a new string with stars.

3: P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Given a recursive implementation, write an iterative implementation, to insert '*' between each character of a given string.

**Recursive Approach: (Provided)**
1) Base Case: If the string length is 1 or less, return the string as is.
2) Recursive Case: Return the first character + '*' + recursive call on the rest of the string.

**Iterative Approach:**
1) If the string length is 1 or less, return the string as is.
2) Initialize an empty result string and add the first character.
3) Iterate over the string from the second character to the end:
   - Append '*' followed by the current character to the result string.
4) Return the result string.

⚠️ Common Mistakes

• Not handling empty strings or single-character strings correctly in either approach.

4: I-mplement

Implement the code to solve the algorithm.

# Recursive approach (Provided)
def insert_stars_recursive(s):
    if len(s) <= 1:
        return s
    else:
        return s[0] + '*' + insert_stars_recursive(s[1:])

# Iterative approach
def insert_stars_iterative(s):
    if len(s) <= 1:
        return s
    result = s[0]
    for char in s[1:]:
        result += '*' + char
    return result

5: R-eview

Review the code by running specific example(s) and recording values (watchlist) of your code's variables along the way.

• Trace through your code with input "hello" to ensure it outputs "hello".
• Validate the edge case with a single character "h" to check that it returns "h" unchanged.

6: E-valuate

Evaluate the performance of your algorithm and state any strong/weak or future potential work.

• Time Complexity: Both methods are O(n) where n is the length of the string, as each character is processed once.
• Space Complexity: O(n) for the recursive approach due to the recursion stack and O(n) for the iterative approach due to the result string size.