feat: Add Dart codes for chapter_dynamic_programming and chapter_greedy (#683)

* feat: Add Dart codes for chapter_dynamic_programming

* feat: Add Dart codes for chapter_greedy

codes/dart/chapter_dynamic_programming/climbing_stairs_dfs.dart

@@ -0,0 +1,27 @@
+/**
+ * File: climbing_stairs_dfs.dart
+ * Created Time: 2023-08-11
+ * Author: liuyuxin (gvenusleo@gmail.com)
+ */
+
+/* 搜索 */
+int dfs(int i) {
+  // 已知 dp[1] 和 dp[2] ，返回之
+  if (i == 1 || i == 2) return i;
+  // dp[i] = dp[i-1] + dp[i-2]
+  int count = dfs(i - 1) + dfs(i - 2);
+  return count;
+}
+
+/* 爬楼梯：搜索 */
+int climbingStairsDFS(int n) {
+  return dfs(n);
+}
+
+/* Driver Code */
+void main() {
+  int n = 9;
+
+  int res = climbingStairsDFS(n);
+  print("爬 $n 阶楼梯共有 $res 种方案");
+}