Heaps

Author: kunal-kushwaha

lectures/24-heaps/code/heaps-1/Heap.java

@@ -0,0 +1,85 @@
+import java.util.ArrayList;
+
+class Heap<T extends Comparable<T>> {
+
+  private ArrayList<T> list;
+
+  public Heap() {
+    list = new ArrayList<>();
+  }
+
+  private void swap(int first, int second) {
+    T temp = list.get(first);
+    list.set(first, list.get(second));
+    list.set(second, temp);
+  }
+
+  private int parent(int index) {
+    return (index - 1) / 2;
+  }
+
+  private int left(int index) {
+    return index * 2 + 1;
+  }
+  
+  private int right(int index) {
+    return index * 2 + 2;
+  }
+
+  public void insert(T value) {
+    list.add(value);
+    upheap(list.size() - 1);
+  }
+  private void upheap(int index) {
+    if(index == 0) {
+      return;
+    }
+    int p = parent(index);
+    if(list.get(index).compareTo(list.get(p)) < 0) {
+      swap(index, p);
+      upheap(p);
+    }
+  }
+
+  public T remove() throws Exception {
+    if (list.isEmpty()) {
+      throw new Exception("Removing from an empty heap!");
+    }
+
+    T temp = list.get(0);
+
+    T last = list.remove(list.size() - 1);
+    if (!list.isEmpty()) {
+      list.set(0, last);
+      downheap(0);
+    }
+    
+    return temp;
+  }
+  private void downheap(int index) {
+    int min = index;
+    int left = left(index);
+    int right = right(index);
+
+    if(left < list.size() && list.get(min).compareTo(list.get(left)) > 0) {
+      min = left;
+    }
+
+    if(right < list.size() && list.get(min).compareTo(list.get(right)) > 0) {
+      min = right;
+    }
+
+    if(min != index) {
+      swap(min, index);
+      downheap(min);
+    }
+  }
+
+  public ArrayList<T> heapSort() throws Exception {
+    ArrayList<T> data = new ArrayList<>();
+    while(!list.isEmpty()) {
+      data.add(this.remove());
+    }
+    return data;
+  }
+}

lectures/24-heaps/code/heaps-1/Main.java

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+import java.util.ArrayList;
+
+class Main {
+  public static void main(String[] args) throws Exception{
+    Heap<Integer> heap = new Heap<>();
+
+    heap.insert(34);
+    heap.insert(45);
+    heap.insert(22);
+    heap.insert(89);
+    heap.insert(76);
+
+    ArrayList list = heap.heapSort();
+    System.out.println(list);
+
+  }
+}