adding test program

lectures/18_trees_I/ds_set_main.cpp

@@ -0,0 +1,32 @@
+#include <iostream>
+#include "ds_set_starter.h"
+// #include <set>
+
+int main() {
+    // create a set of integers
+    std::set<int> numbers;
+
+    // insert some values into the set
+    numbers.insert(10);
+    numbers.insert(5);
+    numbers.insert(20);
+    numbers.insert(15);
+    numbers.insert(5); // Duplicate value (won't be inserted)
+
+    // print the elements of the set
+    std::cout << "The elements in the set are:" << std::endl;
+    for (int num : numbers) {
+        std::cout << num << " ";
+    }
+    std::cout << std::endl;
+
+    // check if a specific value exists in the set
+    int value = 15;
+    if (numbers.find(value) != numbers.end()) {
+        std::cout << value << " is found in the set." << std::endl;
+    } else {
+        std::cout << value << " is not found in the set." << std::endl;
+    }
+
+    return 0;
+}