Minimum Number of Frogs Croaking

Minimum Number of Frogs Croaking

1. What is this problem about?

The Minimum Number of Frogs Croaking interview question involves a string representing the overlapping sounds of multiple frogs. Each frog must croak the full sequence 'c', 'r', 'o', 'a', 'k' in order. You are given a string like "crcoakroak" and must find the minimum number of distinct frogs that could have produced this sound. If the string is invalid, return -1.

2. Why is this asked in interviews?

Roblox and Zoox use this problem to evaluate a candidate's ability to track concurrent events. It's similar to finding the maximum number of overlapping intervals or the peak number of users on a server. It tests state management and the ability to validate a complex sequence in a single pass.

3. Algorithmic pattern used

This problem uses the Counting, String interview pattern. You maintain counters for each character in "croak".

• When you see 'c', a frog starts croaking. (Increase 'c' count, check current active frogs).
• When you see 'r', an active 'c' frog moves to 'r'. (Decrease 'c', increase 'r').
• ...and so on until 'k'.
• When you see 'k', a frog finishes. (Decrease 'a', decrease total active frogs). The answer is the maximum number of frogs that were croaking simultaneously at any point.

4. Example explanation

Input: "crcoakroak"

1. 'c': 1 frog starts.
2. 'r': frog 1 moves to 'r'.
3. 'c': 2nd frog starts. (Active = 2)
4. 'o': 2nd frog moves to 'o'. (Wait, 'r' must happen first... actually, counters track this). If the letters don't follow the sequence (e.g., 'r' appears but 'c' count is 0), it's invalid. The peak number of frogs "in progress" is the result.

5. Common mistakes candidates make

The most common mistake is not checking for invalidity (e.g., characters appearing out of order or characters remaining at the end). Another error is over-complicating the state with a complex object for each frog, when simple counters for each stage ('c', 'r', 'o', 'a') are sufficient. Failing to check if all frogs finished their 'k' at the end is also a frequent oversight.

6. Interview preparation tip

For "sequence tracking" problems, think about the transitions. Each character represents a state change. As long as you have a "source" (the previous character in the sequence), the transition is valid. This pattern is very useful for any problem involving lifecycle tracking.