Refactor MinHeap.

src/data-structures/priority-queue/PriorityQueue.js

@@ -0,0 +1,23 @@
+import MinHeap from '../heap/MinHeap';
+
+// It is the same as min heap except that when comparing to elements
+// we take into account not element's value but rather its priority.
+export default class PriorityQueue extends MinHeap {
+  constructor() {
+    super();
+    this.priorities = {};
+  }
+
+  add(item, priority = 0) {
+    this.priorities[item] = priority;
+    super.add(item);
+  }
+
+  compare(a, b) {
+    if (this.priorities[a] === this.priorities[b]) {
+      return 0;
+    }
+
+    return this.priorities[a] < this.priorities[b] ? -1 : 1;
+  }
+}