Migrate to Null Safety: Fix #176

.gitignore

@@ -19,3 +19,4 @@ doc/api/
 *.js_
 *.js.deps
 *.js.map
+*.json

DIRECTORY.md

@@ -43,8 +43,6 @@
     * [Cycle In Linked List](https://github.com/TheAlgorithms/Dart/blob/master/data_structures/linked_list/cycle_in_linked_list.dart)
     * [Linked List](https://github.com/TheAlgorithms/Dart/blob/master/data_structures/linked_list/linked_list.dart)
     * [Merge Sorted List](https://github.com/TheAlgorithms/Dart/blob/master/data_structures/linked_list/merge_sorted_list.dart)
-  * Quad Tree
-    * [Quad Tree](https://github.com/TheAlgorithms/Dart/blob/master/data_structures/quad_tree/quad_tree.dart)
   * Queue
     * [Circular Queue](https://github.com/TheAlgorithms/Dart/blob/master/data_structures/Queue/Circular_Queue.dart)
     * [List Queue](https://github.com/TheAlgorithms/Dart/blob/master/data_structures/Queue/List_Queue.dart)

array/car_pool.dart

@@ -2,7 +2,7 @@
 // Solution Explanation: https://leetcode.com/problems/car-pooling/solutions/3252690/dart-time-o-n-o-1-space-solution/
 
 import 'dart:math';
-import 'package:test/test.dart';
+import "package:test/test.dart";
 
 bool carPooling(List<List<int>> trips, int capacity) {
   List<int> passengerTimelineCount = List.filled(1001, 0);

conversions/Decimal_To_Any.dart

@@ -48,9 +48,10 @@ String decimalToAny(int value, int base) {
   while (value > 0) {
     int remainder = value % base;
     value = value ~/ base;
-    output =
-        (remainder < 10 ? remainder.toString() : ALPHABET_VALUES[remainder]) +
-            output;
+    output = (remainder < 10
+            ? remainder.toString()
+            : ALPHABET_VALUES[remainder] ?? '0') +
+        output;
   }
 
   return negative ? '-' + output : output;

conversions/Decimal_to_Hexadecimal.dart

@@ -27,7 +27,7 @@ String decimal_to_hexadecimal(int decimal_val) {
     int remainder = decimal_val % 16;
     decimal_val = decimal_val ~/ 16;
     if (hex_table.containsKey(remainder.toString())) {
-      hex_val = hex_table[remainder.toString()];
+      hex_val = hex_table[remainder.toString()] ?? '';
     } else {
       hex_val = remainder.toString();
     }

conversions/Integer_To_Roman.dart

@@ -41,7 +41,7 @@ List<String> RomanNumbers = [
 
 List<String> integer_to_roman(int num) {
   if (num < 0) {
-    return null;
+    return [];
   }
 
   List<String> result = [];

conversions/binary_to_decimal.dart

@@ -4,7 +4,7 @@ import 'package:test/test.dart';
 
 int binaryToDecimal(String binaryString) {
   binaryString = binaryString.trim();
-  if (binaryString == null || binaryString == "") {
+  if (binaryString == "") {
     throw FormatException("An empty value was passed to the function");
   }
   bool isNegative = binaryString[0] == "-";
@@ -14,8 +14,8 @@ int binaryToDecimal(String binaryString) {
     if ("01".contains(binaryString[i]) == false) {
       throw FormatException("Non-binary value was passed to the function");
     } else {
-      decimalValue +=
-          pow(2, binaryString.length - i - 1) * int.parse((binaryString[i]));
+      decimalValue += (pow(2, binaryString.length - i - 1).toInt() *
+          (int.tryParse(binaryString[i]) ?? 0));
     }
   }
   return isNegative ? -1 * decimalValue : decimalValue;

conversions/binary_to_hexadecimal.dart

@@ -24,7 +24,7 @@ Map<String, String> hexTable = {
 String binaryToHexadecimal(String binaryString) {
   // checking for unexpected values
   binaryString = binaryString.trim();
-  if (binaryString == null || binaryString == "") {
+  if (binaryString.isEmpty) {
     throw new FormatException("An empty value was passed to the function");
   }
   try {
@@ -47,7 +47,7 @@ String binaryToHexadecimal(String binaryString) {
   int i = 0;
   while (i != binaryString.length) {
     String bin_curr = binaryString.substring(i, i + 4);
-    hexValue += hexTable[bin_curr];
+    hexValue += hexTable[bin_curr] ?? '';
     i += 4;
   }
 

conversions/binary_to_octal.dart

@@ -21,7 +21,7 @@ void main() {
 
 String binaryToOctal(String binaryString) {
   binaryString = binaryString.trim();
-  if (binaryString == null || binaryString == "") {
+  if (binaryString.isEmpty) {
     throw new FormatException("An empty value was passed to the function");
   }
   bool isNegative = binaryString[0] == "-";

conversions/hexadecimal_to_binary.dart

@@ -24,7 +24,7 @@ Map<String, String> bin_table = {
 String hexadecimal_to_binary(String hex_value) {
   // checking for unexpected values
   hex_value = hex_value.trim();
-  if (hex_value == null || hex_value == "") {
+  if (hex_value.isEmpty) {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -40,7 +40,7 @@ String hexadecimal_to_binary(String hex_value) {
     if (!bin_table.containsKey(hex_cur)) {
       throw new FormatException("An invalid value was passed to the function");
     }
-    bin_val += bin_table[hex_cur];
+    bin_val += bin_table[hex_cur] ?? '';
     i++;
   }
 

conversions/hexadecimal_to_decimal.dart

@@ -21,19 +21,19 @@ void main() {
 
 int hexadecimal_to_decimal(String hex_string) {
   hex_string = hex_string.trim().toUpperCase();
-  if (hex_string == null || hex_string == "") {
+  if (hex_string == "") {
     throw Exception("An empty value was passed to the function");
   }
   bool is_negative = hex_string[0] == "-";
   if (is_negative) hex_string = hex_string.substring(1);
   int decimal_val = 0;
   for (int i = 0; i < hex_string.length; i++) {
-    if (int.parse(hex_string[i], onError: (e) => null) == null &&
+    if (int.tryParse(hex_string[i]) == null &&
         hex_table.containsKey(hex_string[i]) == false) {
       throw Exception("Non-hex value was passed to the function");
     } else {
-      decimal_val += pow(16, hex_string.length - i - 1) *
-          int.parse((hex_string[i]), onError: (e) => hex_table[hex_string[i]]);
+      decimal_val += pow(16, hex_string.length - i - 1).toInt() *
+          (int.tryParse(hex_string[i]) ?? hex_table[hex_string[i]] ?? 0);
     }
   }
   return is_negative ? -1 * decimal_val : decimal_val;

conversions/hexadecimal_to_octal.dart

@@ -6,7 +6,7 @@ String hexadecimal_to_octal(String hex_val) {
   int dec = 0;
 
   // checking for null string passed to function
-  if (hex_val == null || hex_val == "") {
+  if (hex_val == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -29,43 +29,42 @@ String hexadecimal_to_octal(String hex_val) {
       case '7':
       case '8':
       case '9':
-        dec = dec + int.parse(ch) * pow(16, c);
+        dec = dec + (int.tryParse(ch) ?? 0) * pow(16, c).toInt();
         c--;
         break;
       case 'a':
       case 'A':
-        dec = dec + 10 * pow(16, c);
+        dec = dec + 10 * pow(16, c).toInt();
         c--;
         break;
       case 'b':
       case 'B':
-        dec = dec + 11 * pow(16, c);
+        dec = dec + 11 * pow(16, c).toInt();
         c--;
         break;
       case 'c':
       case 'C':
-        dec = dec + 12 * pow(16, c);
+        dec = dec + 12 * pow(16, c).toInt();
         c--;
         break;
       case 'd':
       case 'D':
-        dec = dec + 13 * pow(16, c);
+        dec = dec + 13 * pow(16, c).toInt();
         c--;
         break;
       case 'e':
       case 'E':
-        dec = dec + 14 * pow(16, c);
+        dec = dec + 14 * pow(16, c).toInt();
         c--;
         break;
       case 'f':
       case 'F':
-        dec = dec + 15 * pow(16, c);
+        dec = dec + 15 * pow(16, c).toInt();
         c--;
         break;
       default:
         throw new FormatException(
             "An invalid value was passed to the function");
-        break;
     }
   }
 

conversions/octal_to_binary.dart

@@ -5,7 +5,7 @@ import 'package:test/test.dart';
 String ocatal_to_binary(String oct_val) {
   // checking for unexpected values
   oct_val = oct_val.trim();
-  if (oct_val == null || oct_val == "") {
+  if (oct_val.isEmpty) {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -31,7 +31,7 @@ String ocatal_to_binary(String oct_val) {
   // converting octal to decimal
   int dec_val = 0, i = 0, bin_val = 0;
   while (oct != 0) {
-    dec_val = dec_val + ((oct % 10) * pow(8, i));
+    dec_val = dec_val + ((oct % 10) * pow(8, i).toInt());
     i++;
     oct = oct ~/ 10;
   }

conversions/octal_to_decimal.dart

@@ -7,7 +7,7 @@ import 'package:test/test.dart';
 String ocatal_to_decimal(String oct_val) {
   // checking for unexpected values
   oct_val = oct_val.trim();
-  if (oct_val == null || oct_val == "") {
+  if (oct_val.isEmpty) {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -33,7 +33,7 @@ String ocatal_to_decimal(String oct_val) {
   // converting octal to decimal
   int dec_val = 0, i = 0;
   while (oct != 0) {
-    dec_val = dec_val + ((oct % 10) * pow(8, i));
+    dec_val = dec_val + ((oct % 10) * pow(8, i).toInt());
     i++;
     oct = oct ~/ 10;
   }

conversions/octal_to_hexadecimal.dart

@@ -15,7 +15,7 @@ Map<String, String> hex_table = {
 String ocatal_to_hex(String oct_val) {
   // checking for unexpected values
   oct_val = oct_val.trim();
-  if (oct_val == null || oct_val == "") {
+  if (oct_val == "") {
     throw new FormatException("An empty value was passed to the function");
   }
 
@@ -41,7 +41,7 @@ String ocatal_to_hex(String oct_val) {
   // converting octal to decimal
   int dec_val = 0, i = 0;
   while (oct != 0) {
-    dec_val = dec_val + ((oct % 10) * pow(8, i));
+    dec_val = dec_val + ((oct % 10) * pow(8, i).toInt());
     i++;
     oct = oct ~/ 10;
   }
@@ -56,7 +56,7 @@ String ocatal_to_hex(String oct_val) {
     int remainder = dec_val % 16;
     dec_val = dec_val ~/ 16;
     if (hex_table.containsKey(remainder.toString())) {
-      hex_val = hex_table[remainder.toString()];
+      hex_val = hex_table[remainder.toString()] ?? '';
     } else {
       hex_val = remainder.toString();
     }

data_structures/HashMap/Hashing.dart

@@ -2,8 +2,8 @@
 //Email:[email protected]
 
 class Node {
-  int data;
-  Node next;
+  int? data;
+  Node? next;
 
   Node(int data) {
     this.data = data;
@@ -12,12 +12,11 @@ class Node {
 }
 
 class LinkedList {
-  Node head;
+  Node? head;
   int size;
 
-  LinkedList() {
+  LinkedList({this.size = 0}) {
     head = null;
-    size = 0;
   }
 
   void insert(int data) {
@@ -38,15 +37,15 @@ class LinkedList {
       print("underFlow!");
       return;
     } else {
-      Node curr = head;
-      if (curr.data == data) {
-        head = curr.next;
+      Node? curr = head;
+      if (curr?.data == data) {
+        head = curr?.next;
         size--;
         return;
       } else {
-        while (curr.next.next != null) {
-          if (curr.next.data == data) {
-            curr.next = curr.next.next;
+        while (curr?.next?.next != null) {
+          if (curr?.next?.data == data) {
+            curr?.next = curr.next?.next;
             return;
           }
         }
@@ -56,7 +55,7 @@ class LinkedList {
   }
 
   void display() {
-    Node temp = head;
+    Node? temp = head;
     while (temp != null) {
       print(temp.data.toString());
       temp = temp.next;
@@ -69,8 +68,8 @@ class HashMap {
   int hsize;
   List<LinkedList> buckets;
 
-  HashMap(int hsize) {
-    buckets = new List<LinkedList>(hsize);
+  HashMap({this.hsize = 0, this.buckets = const []}) {
+    buckets = new List.generate(hsize, (a) => LinkedList());
     for (int i = 0; i < hsize; i++) {
       buckets[i] = new LinkedList();
     }
@@ -104,7 +103,7 @@ class HashMap {
 }
 
 void main() {
-  HashMap h = new HashMap(7);
+  HashMap h = new HashMap(hsize: 7);
 
   print("Add key 5");
   h.insertHash(5);

data_structures/Heap/Binary_Heap/Max_heap.dart

@@ -1,7 +1,7 @@
 import 'package:test/test.dart';
 
 class MaxHeap {
-  List<int> heap;
+  List<int> heap = [];
 
   void buildHeap(List<int> array) {
     this.heap = _heapify(array);
@@ -15,7 +15,7 @@ class MaxHeap {
     return array;
   }
 
-  int peek() {
+  int? peek() {
     if (!isEmpty()) {
       return this.heap[0];
     }
@@ -65,7 +65,7 @@ class MaxHeap {
     _siftUp(this.heap.length - 1);
   }
 
-  int remove() {
+  int? remove() {
     if (!isEmpty()) {
       _swap(0, this.heap.length - 1, this.heap);
       int maxElement = this.heap.removeLast();

data_structures/Heap/Binary_Heap/min_heap_two.dart

@@ -1,7 +1,7 @@
 import 'package:test/test.dart';
 
 class MinHeap {
-  List<int> heap;
+  List<int> heap = [];
 
   void buildHeap(List<int> array) {
     this.heap = _heapify(array);
@@ -15,7 +15,7 @@ class MinHeap {
     return array;
   }
 
-  int peek() {
+  int? peek() {
     if (!isEmpty()) {
       return this.heap[0];
     }
@@ -65,7 +65,7 @@ class MinHeap {
     _siftUp(this.heap.length - 1);
   }
 
-  int remove() {
+  int? remove() {
     if (!isEmpty()) {
       _swap(0, this.heap.length - 1, this.heap);
       int minElement = this.heap.removeLast();