# Rate Limiter - Amazon Top Interview Questions

### Problem Statement :

```Implement a rate limiter that limits users’ requests with the following methods:

RateLimiter(int expire) constructs a new rate limiter with the given expire time.
limit(int uid, int timestamp) represents a request from user uid at time timestamp and should return whether the given user’s request fails. It should fail if the user had a successful request less than expire time ago.
You can assume that timestamp is monotonically increasing between requests.

Constraints

n ≤ 100,000 where n is the number of calls to limit.

Example 1

Input

methods = ["constructor", "limit", "limit", "limit", "limit"]

arguments = [[5], [0, 10], [0, 15], [0, 16], [1, 17]]`

Output

[None, False, False, True, False]

Explanation

rl = RateLimiter(5)

rl.limit(0, 10) == False

rl.limit(0, 15) == False

rl.limit(0, 16) == True

rl.limit(1, 17) == False

Example 2

Input

methods = ["constructor", "limit", "limit", "limit", "limit"]

arguments = [[0], [0, 1], [0, 1], [0, 2], [0, 3]]`

Output

[None, False, False, False, False]

Explanation

rl = RateLimiter(0)

rl.limit(0, 1) == False

rl.limit(0, 1) == False

rl.limit(0, 2) == False

rl.limit(1, 3) == False

Example 3

Input

methods = ["constructor", "limit", "limit", "limit"]

arguments = [[5], [0, 10], [0, 13], [0, 16]]`

Output

[None, False, True, False]

Explanation

rl = RateLimiter(5)

rl.limit(0, 10) == False

rl.limit(0, 13) == True

rl.limit(0, 16) == False```

### Solution :

```                        ```Solution in C++ :

class RateLimiter {
public:
RateLimiter(int expire) : limitThreshold(expire) {
}  // Remember the limit value

bool limit(int uid, int timestamp) {
// If the new request came later than minimum allowed we return false and create a new
// minimum allowed request time
if (timestamp >= lastTime[uid]) {
lastTime[uid] = timestamp + limitThreshold;
return false;
}
return true;  // Reject the request
}

private:
unordered_map<int, int> lastTime;  // Storage of uid and minimum allowed request time
// (initialized to 0 when the first requests comes)
int limitThreshold;                // Limit value
};```
```

```                        ```Solution in Java :

import java.util.*;

class RateLimiter {
public int expire;
public HashMap<Integer, Integer> hm = new HashMap<>();
public RateLimiter(int expire) {
this.expire = expire;
}

public boolean limit(int uid, int timestamp) {
if (hm.containsKey(uid)) {
if (hm.get(uid) <= timestamp) {
hm.put(uid, timestamp + expire);
return false;
}
return true;

} else {
hm.put(uid, timestamp + expire);
return false;
}
}
}```
```

```                        ```Solution in Python :

class RateLimiter:
def __init__(self, expire):
self.expire = expire
self.lastCall = defaultdict(lambda: -1)

def limit(self, uid, timestamp):
last = self.lastCall[uid]
if last == -1 or last + self.expire <= timestamp:
self.lastCall[uid] = timestamp
return False
return True```
```

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