feat: Add Dart codes for chapter_backtracking and chapter_divide_and_conquer (#680)

* feat: Add Dart codes for chapter_backtracking

* feat: Add Dart codes for chapter_divide_and_conquer

* Format Dart Code

codes/dart/chapter_backtracking/subset_sum_i_naive.dart

@@ -0,0 +1,54 @@
+/**
+ * File: subset_sum_i_naive.dart
+ * Created Time: 2023-08-10
+ * Author: liuyuxin (gvenusleo@gmail.com)
+ */
+
+/* 回溯算法：子集和 I */
+void backtrack(
+  List<int> state,
+  int target,
+  int total,
+  List<int> choices,
+  List<List<int>> res,
+) {
+  // 子集和等于 target 时，记录解
+  if (total == target) {
+    res.add(List.from(state));
+    return;
+  }
+  // 遍历所有选择
+  for (int i = 0; i < choices.length; i++) {
+    // 剪枝：若子集和超过 target ，则跳过该选择
+    if (total + choices[i] > target) {
+      continue;
+    }
+    // 尝试：做出选择，更新元素和 total
+    state.add(choices[i]);
+    // 进行下一轮选择
+    backtrack(state, target, total + choices[i], choices, res);
+    // 回退：撤销选择，恢复到之前的状态
+    state.removeLast();
+  }
+}
+
+/* 求解子集和 I（包含重复子集） */
+List<List<int>> subsetSumINaive(List<int> nums, int target) {
+  List<int> state = []; // 状态（子集）
+  int total = 0; // 元素和
+  List<List<int>> res = []; // 结果列表（子集列表）
+  backtrack(state, target, total, nums, res);
+  return res;
+}
+
+/* Driver Code */
+void main() {
+  List<int> nums = [3, 4, 5];
+  int target = 9;
+
+  List<List<int>> res = subsetSumINaive(nums, target);
+
+  print("输入数组 nums = $nums, target = $target");
+  print("所有和等于 $target 的子集 res = $res");
+  print("请注意，该方法输出的结果包含重复集合");
+}