Implemented the "Heap" data structure

lib/data_structures/heap.dart

@@ -0,0 +1,83 @@
+class Heap<T> {
+
+  final int Function(T) getPriority;
+  List<(T, int)> _items;
+
+  Heap({
+    required this.getPriority
+  }): _items = [];
+
+  factory Heap.fromItems(
+    List<T> items,
+    {
+      required int Function(T) getPriority,
+    }
+  ) {
+    Heap<T> heap = Heap(getPriority: getPriority);
+    for (final T item in items) {
+      heap.add(item);
+    }
+    return heap;
+  }
+
+  T get first => _items.first.$1;
+
+  T? get firstOrNull => _items.firstOrNull?.$1;
+
+  int get length => _items.length;
+
+  bool get isEmpty => _items.isEmpty;
+
+  bool get isNotEmpty => _items.isNotEmpty;
+
+  void add(T item) {
+    final (T, int) heapItem = (item, getPriority(item));
+    final int thisKey = heapItem.$2;
+    int i = _items.length;
+    _items.add(heapItem);
+    while (i > 0 && _items[(i-1) ~/ 2].$2 > thisKey) {
+      _items[i] = _items[(i-1) ~/ 2];
+      i = (i-1)~/2;
+    }
+    _items[i] = heapItem;
+  }
+
+  T removeFirst() {
+    if (_items.isEmpty) {
+      throw StateError("No element");
+    }
+    final T firstItem = _items[0].$1;
+    if (_items.length < 2) {
+      _items.length--;
+      return firstItem;
+    }
+    final (T, int) itemToMove = _items.last;
+    _items[0] = itemToMove;
+    _items.length--;
+    int i = 0;
+    int leftIndex, rightIndex;
+    bool stop = false;
+    while (!stop) {
+      rightIndex = (i+1) << 1;
+      leftIndex = rightIndex-1;
+      if (leftIndex < _items.length && itemToMove.$2 > _items[leftIndex].$2) {
+        if (rightIndex < _items.length && _items[leftIndex].$2 > _items[rightIndex].$2) {
+          _items[i] = _items[rightIndex];
+          _items[rightIndex] = itemToMove;
+          i = rightIndex;
+        } else {
+          _items[i] = _items[leftIndex];
+          _items[leftIndex] = itemToMove;
+          i = leftIndex;
+        }
+      } else {
+        stop = true;
+      }
+    }
+    return firstItem;
+  }
+
+  @override
+  String toString() => _items.map(((T, int) item) => item.$1).toString();
+
+}