hit counter done

Author: rampatra

src/main/java/com/leetcode/design/DesignHitCounter.java

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+package com.leetcode.design;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Level: Medium
+ * Problem Link: https://leetcode.com/problems/design-hit-counter/
+ * Problem Description:
+ * Design a hit counter which counts the number of hits received in the past 5 minutes.
+ *
+ * Each function accepts a timestamp parameter (in seconds granularity) and you may assume that calls are being made
+ * to the system in chronological order (ie, the timestamp is monotonically increasing). You may assume that the
+ * earliest timestamp starts at 1.
+ *
+ * It is possible that several hits arrive roughly at the same time.
+ *
+ * Example:
+ *
+ * HitCounter counter = new HitCounter();
+ *
+ * // hit at timestamp 1.
+ * counter.hit(1);
+ *
+ * // hit at timestamp 2.
+ * counter.hit(2);
+ *
+ * // hit at timestamp 3.
+ * counter.hit(3);
+ *
+ * // get hits at timestamp 4, should return 3.
+ * counter.getHits(4);
+ *
+ * // hit at timestamp 300.
+ * counter.hit(300);
+ *
+ * // get hits at timestamp 300, should return 4.
+ * counter.getHits(300);
+ *
+ * // get hits at timestamp 301, should return 3.
+ * counter.getHits(301);
+ *
+ * Follow up:
+ * What if the number of hits per second could be very large? Does your design scale?
+ *
+ * Runtime: <a href="https://leetcode.com/submissions/detail/248917167/">42 ms</a> (better than ~98%).
+ *
+ * @author rampatra
+ * @since 2019-08-04
+ */
+public class DesignHitCounter {
+
+    private int[] timestamps;
+    private int[] hits;
+
+    /**
+     * Initialize your data structure here.
+     */
+    public DesignHitCounter() {
+        this.timestamps = new int[300];
+        this.hits = new int[300];
+    }
+
+    /**
+     * Record a hit.
+     *
+     * @param timestamp - The current timestamp (in seconds granularity).
+     */
+    public void hit(int timestamp) {
+        int bucket = timestamp % 300;
+        if (timestamps[bucket] == timestamp) {
+            hits[bucket]++;
+        } else {
+            timestamps[bucket] = timestamp;
+            hits[bucket] = 1;
+        }
+    }
+
+    /**
+     * Return the number of hits in the past 5 minutes.
+     *
+     * @param timestamp - The current timestamp (in seconds granularity).
+     */
+    public int getHits(int timestamp) {
+        int totalHits = 0;
+        for (int i = 0; i < 300; i++) {
+            if (timestamp - 300 < timestamps[i]) {
+                totalHits += hits[i];
+            }
+        }
+        return totalHits;
+    }
+
+    public static void main(String[] args) {
+        DesignHitCounter counter = new DesignHitCounter();
+
+        // hit at timestamp 1.
+        counter.hit(1);
+
+        // hit at timestamp 2.
+        counter.hit(2);
+
+        // hit at timestamp 3.
+        counter.hit(3);
+
+        // get hits at timestamp 4, should return 3.
+        assertEquals(3, counter.getHits(4));
+
+        // hit at timestamp 300.
+        counter.hit(300);
+
+        // get hits at timestamp 300, should return 4.
+        assertEquals(4, counter.getHits(300));
+
+        // get hits at timestamp 301, should return 3.
+        assertEquals(3, counter.getHits(301));
+    }
+}