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One natural approach to solve this problem is to use a queue data structure. A queue is perfect for this task because it works on a first-in-first-out (FIFO) principle. When a new request arrives, we add it to the queue. We also remove requests from the front of the queue that are older than 3000 milliseconds compared to the current time t. This allows us to keep only the requests within the last 3000 milliseconds in the queue. The number of such requests is simply the size of the queue after cleaning.
Time Complexity: Amortized O(1) because each request can only be removed once from the queue.
Space Complexity: O(W), where W is the maximum number of recent requests within 3000 milliseconds.
1#include <stdio.h>
2#include <stdlib.h>
3
4#define MAX_REQUESTS 10000
5
6typedef struct {
7 int requests[MAX_REQUESTS];
8 int front;
9 int rear;
10} RecentCounter;
11
12RecentCounter* recentCounterCreate() {
13 RecentCounter *obj = (RecentCounter *)malloc(sizeof(RecentCounter));
14 obj->front = 0;
15 obj->rear = 0;
16 return obj;
17}
18
19int ping(RecentCounter* obj, int t) {
20 obj->requests[obj->rear++] = t;
21 while (obj->requests[obj->front] < t - 3000) {
22 obj->front++;
23 }
24 return obj->rear - obj->front;
25}
26
27void recentCounterFree(RecentCounter* obj) {
28 free(obj);
29}This solution uses a simple array to act as a queue with two pointers: front and rear. New requests are added to the rear of the queue. Outdated requests, those older than 3000 milliseconds, are removed by incrementing the front pointer. This ensures the time complexity of each ping operation is amortized O(1).
An alternative approach to using a queue is to use a double-ended queue (deque). While similar to a queue, a deque allows us to add and remove elements from both ends, but in this problem, we'll only need the standard queue operations.
The logic remains the same: insert new request times to the back of the deque and remove outdated ones from the front. This keeps the deque maintaining only the relevant requests within the [t-3000, t] range.
Time Complexity: Amortized O(1) for each ping due to efficient pop and push operations.
Space Complexity: O(W), where W is size of the time window.
1#include <deque>
2using namespace std;
3
class RecentCounter {
deque<int> deq;
public:
RecentCounter() {}
int ping(int t) {
deq.push_back(t);
while (!deq.empty() && deq.front() < t - 3000) {
deq.pop_front();
}
return deq.size();
}
};This C++ implementation uses a deque from the standard library to manage timestamps. Similar to a queue, we check and remove outdated times once each new ping is added.