top k frequent elements

Author: JisooPyo

top-k-frequent-elements/JisooPyo.kt

@@ -0,0 +1,76 @@
+package leetcode_study
+
+import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.Test
+import java.util.*
+
+/**
+ * Leetcode
+ * 347. Top K Frequent Elements
+ * Medium
+ */
+class TopKFrequentElements {
+    /**
+     * Runtime: 30 ms(Beats: 68.62 %)
+     * Time Complexity: O(n log n)
+     *   - list 정렬
+     *
+     * Memory: 42.20 MB(Beats: 58.82 %)
+     * Space Complexity: O(n)
+     */
+    fun topKFrequent(nums: IntArray, k: Int): IntArray {
+        val countMap: MutableMap<Int, Int> = HashMap()
+
+        for (num in nums) {
+            countMap[num] = countMap.getOrDefault(num, 0) + 1
+        }
+
+        val list = mutableListOf<Node>()
+        for (key in countMap.keys) {
+            list.add(Node(key, countMap[key]!!))
+        }
+        list.sortDescending()
+
+        val answer = IntArray(k)
+        for (i in 0 until k) {
+            answer[i] = list[i].value
+        }
+        return answer
+    }
+
+    /**
+     * 개선된 버전: 우선순위 큐를 사용
+     *
+     * Runtime: 19 ms(Beats: 96.30 %)
+     * Time Complexity: O(n log n)
+     *
+     * Memory: 44.83 MB(Beats: 18.35 %)
+     * Space Complexity: O(n)
+     */
+    fun topKFrequent2(nums: IntArray, k: Int): IntArray {
+        val countMap = nums.groupBy { it }
+            .mapValues { it.value.size }
+
+        val pq = PriorityQueue<Node>(compareByDescending { it.count })
+        countMap.forEach { (num, count) ->
+            pq.offer(Node(num, count))
+        }
+
+        return IntArray(k) { pq.poll().value }
+    }
+
+    data class Node(var value: Int, var count: Int) : Comparable<Node> {
+        override fun compareTo(other: Node): Int {
+            return this.count.compareTo(other.count)
+        }
+    }
+
+    @Test
+    fun test() {
+        topKFrequent(intArrayOf(1, 1, 1, 2, 2, 3), 2) shouldBe intArrayOf(1, 2)
+        topKFrequent(intArrayOf(1), 1) shouldBe intArrayOf(1)
+
+        topKFrequent2(intArrayOf(1, 1, 1, 2, 2, 3), 2) shouldBe intArrayOf(1, 2)
+        topKFrequent2(intArrayOf(1), 1) shouldBe intArrayOf(1)
+    }
+}