Coding

Logger Rate Limiter

Asked at

Microsoft

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

A logger system receives a stream of messages along with their timestamps. Each unique message should be printed at most once every 10 seconds. If a message is printed at timestamp t, the same message cannot be printed again until timestamp t + 10 or later.

All messages arrive in chronological order, and multiple messages may arrive at the same timestamp.

Implement the Logger class:

**Logger(): **Initializes the logger object.
**bool shouldPrintMessage(int timestamp, string message): **Returns true if the message should be printed at the given timestamp, otherwise returns false.

Input: ["Logger", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage"] [[], [1, "apple"], [3, "banana"], [5, "apple"], [12, "apple"]] Output: [null, true, true, false, true]

Input: ["Logger", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage"] [[], [2, "hello"], [4, "hello"], [12, "hello"], [15, "world"]] Output: [null, true, false, true, true]

Input: ["Logger", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage", "shouldPrintMessage"] [[], [1, "test"], [9, "test"], [10, "test"], [11, "test"]]

0 <= timestamp <= 109
Every timestamp is given in non-decreasing order (chronological order).
1 <= message.length <= 30
At most 104 calls will be made to shouldPrintMessage.

cpp

class Logger {
public:
    Logger() {
        // Your code goes here
    }

    bool shouldPrintMessage(int timestamp, string message) {
        // Your code goes here
    }
};

java

class Logger {
    public Logger() {
        // Your code goes here
    }

    public boolean shouldPrintMessage(int timestamp, String message) {
        // Your code goes here
    }
}

python

class Logger:
    def __init__(self):
        # Your code goes here

    def shouldPrintMessage(self, timestamp: int, message: str) -> bool:
        # Your code goes here

javascript

class Logger {
    constructor() {
        // Your code goes here
    }

    shouldPrintMessage(timestamp, message) {
        // Your code goes here
    }
}

csharp

public class Logger
{
    public Logger()
    {
        // Your code goes here
    }

    public bool ShouldPrintMessage(int timestamp, string message)
    {
        // Your code goes here
        return false;
    }
}

type Logger struct {
	// Your fields here
	lastPrinted map[string]int
}

func Constructor() Logger {
	// Your initialization here
	return Logger{
		lastPrinted: make(map[string]int),
	}
}

func (this *Logger) ShouldPrintMessage(timestamp int, message string) bool {
	// Your code here
	return false
}

Stuck? Show a way to structure it+

01Clarify whether timestamps are monotonic
02Look up the message's next allowed time
03Reject if current time is earlier
04On acceptance store timestamp plus the window

Reference answer

Start by restating the input contract for Logger Rate Limiter and testing the smallest boundary case before choosing the algorithm. Keep a hash map from message text to the earliest timestamp at which it may print again. For an incoming (timestamp, message), print only when the message is absent or timestamp is at least its stored next-allowed time, then set next-allowed to timestamp plus 10. Explain the invariant while coding, then show how each update preserves it. Each call is expected O(1) time; the map uses O(u) space for u distinct messages retained. Walk through one nontrivial example aloud so the interviewer can see why the method reaches the intended result. Finally, call out input assumptions and failure cases rather than silently relying on them. A clean implementation should keep state local, use descriptive names, and avoid work that the invariant makes unnecessary.

Then expect these follow-ups

How can expired keys be cleaned up?
Tests: follow-up reasoning
How would per-user limits change the key?
Tests: constraint adaptation

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