Dart version of tree.ts

tree.dart

// tree.ts https://gist.github.com/dmitry-vsl/ab40c3b325bcf0a0eb07

// Run it in Dartpad: https://dartpad.dartlang.org/1a4a46dce98278473255

class Tree<T> {
  final Tree<T> left;
  final Tree<T> right;
  final T value;

  Tree(this.left, this.right, this.value);

  factory Tree.fromMap(Map map) {
    if (map == null) return null;

    assert(map.containsKey('left'));
    assert(map.containsKey('right'));
    assert(map.containsKey('value'));

    return new Tree<T>(
      new Tree.fromMap(map['left']),
      new Tree.fromMap(map['right']),
      map['value']
    );
  }

  Map toMap() => {
      'left': left?.toMap(),
      'right': right?.toMap(),
      'value': value
  };

  // We are losing type of "a" here.
  // https://github.com/dart-lang/dart_enhancement_proposals/issues/22
  reduce(initial, reducer(a, T b)) {
    var withNodeValue = reducer(initial, value);
    var withLeftBranch = left?.reduce(withNodeValue, reducer);
    var withRightBranch = right?.reduce(withLeftBranch ?? withNodeValue, reducer);
    return withRightBranch ?? withNodeValue;
  }
}

num average(Tree<num> tree) {
  var reducer = (averageAcc, num val) => {
    'sum': averageAcc['sum'] + val,
    'count': averageAcc['count'] + 1
  };

  var result = tree.reduce({'sum': 0, 'count': 0}, reducer);
  return result['sum'] / result['count'];
}

main() {
  var treeMap = {
  	'value': 4,
  	'left': {
  		'value': 5,
  		'left': null,
  		'right': null
  	},
  	'right': {
  		'value': 1,
  		'left': null,
  		'right': {
  			'value': 2,
  			'left': null,
  			'right': null
  		}
  	},
  };

  var tree = new Tree<num>.fromMap(treeMap);
  
  print(average(tree));
}