feat(soobing): week1 > top-k-frequent-elements

Author: soobing

top-k-frequent-elements/soobing2.ts

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+/**
+ * 문제 유형: Heap
+ *
+ * 문제 설명
+ * - 가장 빈도수가 많은 K개 Element 추출
+ *
+ * 아이디어
+ * 1) 빈도수 계산하여 index와 함께 Map에 저장, 빈도수를 기준으로 MinHeap 구성
+ * 2) 빈도수 Map을 순회하면서 MinHeap에 추가 + MinHeap의 크기가 K를 초과하면 최소값 제거 (미리미리 빈도수가 많은 K개 Element를 만들어나가는 형식)
+ * 3) MinHeap에 남아있는 Element들의 index를 반환
+ *
+ * 시간 복잡도: O(NlogK)
+ * 공간 복잡도: O(N)
+ */
+class MinHeap {
+  heap: [number, number][] = [];
+
+  insert(item: [number, number]) {
+    this.heap.push(item);
+    this.bubbleUp();
+  }
+
+  private bubbleUp() {
+    let index = this.heap.length - 1;
+    while (index > 0) {
+      const parentIndex = Math.floor((index - 1) / 2);
+      if (this.heap[index][0] >= this.heap[parentIndex][0]) break;
+      [this.heap[index], this.heap[parentIndex]] = [
+        this.heap[parentIndex],
+        this.heap[index],
+      ];
+      index = parentIndex;
+    }
+  }
+
+  extractMin(): [number, number] | undefined {
+    if (this.heap.length === 0) return undefined;
+    const min = this.heap[0];
+    const end = this.heap.pop();
+    if (this.heap.length > 0 && end !== undefined) {
+      this.heap[0] = end;
+      this.sinkDown(0);
+    }
+    return min;
+  }
+
+  private sinkDown(index: number) {
+    const length = this.heap.length;
+    while (true) {
+      let left = 2 * index + 1;
+      let right = 2 * index + 2;
+      let smallest = index;
+
+      if (left < length && this.heap[left][0] < this.heap[smallest][0]) {
+        smallest = left;
+      }
+      if (right < length && this.heap[right][0] < this.heap[smallest][0]) {
+        smallest = right;
+      }
+      if (index === smallest) break;
+
+      [this.heap[index], this.heap[smallest]] = [
+        this.heap[smallest],
+        this.heap[index],
+      ];
+      index = smallest;
+    }
+  }
+
+  peek(): [number, number] | undefined {
+    return this.heap[0];
+  }
+
+  size(): number {
+    return this.heap.length;
+  }
+}
+function topKFrequent(nums: number[], k: number): number[] {
+  const freqMap = new Map<number, number>();
+  for (const num of nums) {
+    freqMap.set(num, (freqMap.get(num) ?? 0) + 1);
+  }
+
+  const minHeap = new MinHeap();
+  for (const [num, count] of freqMap.entries()) {
+    minHeap.insert([count, num]);
+    if (minHeap.size() > k) {
+      minHeap.extractMin();
+    }
+  }
+
+  return minHeap.heap.map(([_, num]) => num);
+}