Substring

Problem

Write a function that takes in two strings and returns true if the second string is substring of the first, and false otherwise.

Problem Highlights

• 🔗 Leetcode Link: n/a
• 💡 Problem Difficulty: Medium
• ⏰ Time to complete: __ mins
• 🛠️ Topics: Strings
• 🗒️ Similar Questions: TBD

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?

• Are both inputs only strings?
  • Can elements of the string be both characters and numbers?
• Can the second string be larger than the first?
• What are the Time & Space considerations?
  • Best Case Time Complexity: O(n * m)
  • Best Case Space Complexity: O(1)

Run through a set of example cases:

HAPPY CASE
Input: laboratory, rat
Output: true

Input: cat, meow
Output: false

EDGE CASE
Input: "CATDOG", "
Output: True

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.

For Strings, common solution patterns include:

• Two pointer solutions (left and right pointer variables) or a pointer on each string
• Storing the characters of the string in a HashMap or a Set
• Traversing the string with a sliding window

3: P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Loop through the first string for an initial match. If we find one, check that match for a full substring match.

1) Edge Check
2) Loop through the first string
3) Find a match to the first character of the second string
4) While there is a match, loop through both strings in parallel to ensure 
    the match
5) If there is an entire match to the second string, return True (there is a 
    substring)
6) If the match breaks, continue searching the first string for an initial 
    character match
7) If we make it to the end of the first string without a full match, there 
    is no substring match -- return False 

⚠️ Common Mistakes

• Some students may want to hash components of the strings, but this does not preserve order.
• We can solve this problem without any extra allocated space.

4: I-mplement

Implement the code to solve the algorithm.

def substring(large_str, potential_substr):
    substr_len = len(potential_substr)

    # iterate through larger str for its length minus substr length
    for i in range(len(large_str) - substr_len):
        # iterate through potential substr  
        for j in range(substr_len):
            # if chars match in both string
            if large_str[i + j] == potential_substr[j]: 
                # if at the end of substring, return true
                if j == (substr_len - 1):
                    return True
                # else do nothing and continue looping
            else:
                break

    # did not find substring
    return False

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 an input to check for the expected output
• Catch possible edge cases and off-by-one errors

6: E-valuate

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

• Time Complexity: O(n*m), where n: length of first string, m: length of second string
• Space Complexity: O(1), where no data structures are needed in this problem