diff --git a/lesson3/heap.cpp b/lesson3/heap.cpp
new file mode 100644
index 00000000..307a8c1c
--- /dev/null
+++ b/lesson3/heap.cpp
@@ -0,0 +1,19 @@
+#include <vector>
+//#include <priority_queue>
+
+// left child: 2*i + 1;
+// right child: 2*i + 2;
+// parent: [(i-1)/2];
+
+struct Heap {
+  Heap() : size{0}, values{} {}
+
+  void sift_up(int i);
+  void sift_down(int i);
+  void append(int x);
+
+  std::vector<int> values;
+  int size;
+};
+
+void Heap::sift_up(int i) {}
\ No newline at end of file
diff --git a/lesson3/sorting_algorithms.cpp b/lesson3/sorting_algorithms.cpp
new file mode 100644
index 00000000..1b0610d9
--- /dev/null
+++ b/lesson3/sorting_algorithms.cpp
@@ -0,0 +1,21 @@
+#include <vector>
+
+// bubble_sort: O*(n^2), O(n^2)
+// bogo_sort: O*(n!), O(+inf)
+// insertion_sort: O*(n^2), O(n^2)
+// selection_sort: O*(n^2), O(n^2)
+// merge_sort (top_down/bottom_up): O*(nlogn), O(nlogn)
+// quick_sort: O*(nlogn), (O(n^2) - ineffective, O(nlogn) - effective)
+
+// qsort [] = []
+// qsort(x : xs) = qsort(filter(<x) xs) ++ [x] (filter(=x) xs) ++
+// qsort(filter(>x) xs)
+
+void bubble_sort(std::vector<int>& v) {}
+
+void td_merge_sort(std::vector<int>& v) {
+  for (int l = 1; l < v.size(); l++) {
+    for (int i = 0; i + l < v.size(); i += 2 * l) {
+    }
+  }
+}
\ No newline at end of file
diff --git a/lesson5/search_tree.cpp b/lesson5/search_tree.cpp
new file mode 100644
index 00000000..8f66d502
--- /dev/null
+++ b/lesson5/search_tree.cpp
@@ -0,0 +1,62 @@
+#include "search_tree.hpp"
+
+#include <stdexcept>
+
+void Node::add_left_node(std::string data_, int key_) {
+  left_child = new Node(data_, key_, this);
+}
+
+void Node::add_right_node(std::string data_, int key_) {
+  right_child = new Node(data_, key_, this);
+}
+
+void Node_swap(Node* node1, Node* node2) {
+  std::swap(node1->data, node2->data);
+  std::swap(node1->key, node2->key);
+}
+
+std::string& Tree::Find(Node* current_node, int key) {
+  if (current_node == nullptr) throw std::runtime_error("No such key");
+  if (current_node->key == key)
+    return current_node->data;
+  else if (key < current_node->key)
+    return Find(current_node->left_child, key);
+  else
+    return Find(current_node->right_child, key);
+}
+
+std::pair<std::string, Node*> Tree::Find_Node(Node* current_node, int key) {
+  if (current_node == nullptr) return {"no head", current_node};
+  if (current_node->key == key)
+    return {"repeat", current_node};
+  else if (key < current_node->key) {
+    if (current_node->left_child == nullptr) return {"left", current_node};
+    return Find_Node(current_node->left_child, key);
+  } else {
+    if (current_node->right_child == nullptr) return {"right", current_node};
+    return Find_Node(current_node->right_child, key);
+  }
+}
+
+bool Tree::Add(int key, std::string data_) {
+  std::pair<std::string, Node*> needed_node_pair = Find_Node(head, key);
+  if (needed_node_pair.first == "no head") {
+    head = new Node(data_, key);
+    return true;
+  } else if (needed_node_pair.first == "repeat")
+    return false;
+  else if (needed_node_pair.first == "left") {
+    needed_node_pair.second->left_child =
+        new Node(data_, key, needed_node_pair.second);
+    return true;
+  } else if (needed_node_pair.first == "right") {
+    needed_node_pair.second->left_child =
+        new Node(data_, key, needed_node_pair.second);
+    return true;
+  }
+  throw std::runtime_error(
+      "Tree::Add(...) function reaches the end without returning a value, fix "
+      "this");
+}
+
+std::string& Tree::Remove(int key) {}
\ No newline at end of file
diff --git a/lesson5/search_tree.hpp b/lesson5/search_tree.hpp
new file mode 100644
index 00000000..fb56cec0
--- /dev/null
+++ b/lesson5/search_tree.hpp
@@ -0,0 +1,54 @@
+#include <algorithm>
+#include <functional>
+#include <optional>
+#include <stdexcept>
+#include <string>
+
+class Summator {
+  int sum;
+  void operator()(int, std::string);
+};
+
+struct Node {
+  Node()
+      : parent{nullptr},
+        left_child{nullptr},
+        right_child{nullptr},
+        data{""},
+        key{0} {}
+  Node(std::string data_, int key_)
+      : parent{nullptr},
+        left_child{nullptr},
+        right_child{nullptr},
+        data{data_},
+        key{key_} {}
+  Node(std::string data_, int key_, Node* parent_)
+      : parent{parent_},
+        left_child{nullptr},
+        right_child{nullptr},
+        data{data_},
+        key{key_} {}
+  void add_left_node(std::string data_, int key_);
+  void add_right_node(std::string data_, int key_);
+  Node* parent;
+  Node* left_child;
+  Node* right_child;
+  int key;
+  std::string data;
+};
+
+class Tree {
+ public:
+  Tree() : head{nullptr} {}
+
+  bool Add(int key, std::string data_);
+  // could be replaced with "std::optional<std::string>"
+  std::string& Find(Node* current_node, int key);
+  std::string& Remove(int key);
+  void ForEach(std::function<void(int, std::string)>);
+
+ private:
+  std::pair<std::string, Node*> Find_Node(Node* current_node, int key);
+
+  Node* head;
+};
\ No newline at end of file
diff --git a/sandbox/template/src/main b/sandbox/template/src/main
new file mode 100755
index 00000000..9342dd8d
Binary files /dev/null and b/sandbox/template/src/main differ
diff --git a/sandbox/template/src/main.cpp b/sandbox/template/src/main.cpp
index 0e4393ba..0a554f65 100644
--- a/sandbox/template/src/main.cpp
+++ b/sandbox/template/src/main.cpp
@@ -1,3 +1,6 @@
 #include <iostream>
 
-int main() { return 0; }
+int main() {
+  std::cout << "Hello world!";
+  return 0;
+}
diff --git a/task_01/src/main b/task_01/src/main
new file mode 100755
index 00000000..c3c1e2a9
Binary files /dev/null and b/task_01/src/main differ
diff --git a/task_01/src/main.cpp b/task_01/src/main.cpp
index 0e4393ba..7825a349 100644
--- a/task_01/src/main.cpp
+++ b/task_01/src/main.cpp
@@ -1,3 +1,22 @@
 #include <iostream>
 
-int main() { return 0; }
+#include "solution.h"
+
+int main() {
+  int sum, arr_size, t;
+  std::vector<int> input_vector;
+  std::unordered_map<int, int> indices_map;  // keeps array numbers as keys and
+                                             // indices as values behind keys
+
+  std::cin >> sum >> arr_size;
+
+  for (int i = 0; i < arr_size; i++) {
+    std::cin >> t;
+    input_vector.push_back(t);
+  }
+
+  std::pair<int, int> sol = solution(sum, input_vector);
+  std::cout << sol.first << ' ' << sol.second;
+
+  return 0;
+}
\ No newline at end of file
diff --git a/task_01/src/solution.h b/task_01/src/solution.h
new file mode 100644
index 00000000..c423e0c5
--- /dev/null
+++ b/task_01/src/solution.h
@@ -0,0 +1,41 @@
+#include <unordered_map>
+#include <vector>
+
+/*
+
+Output - indices of two numbers, which sum is equal to needed number, if there's
+no such numbers, the output is "-1 -1"
+
+Input:
+
+10
+10
+-2 2 3 3 5 8 11 13 14 15
+
+Output:
+
+1 5
+
+*/
+
+// Solution below has a time complexity of O(n) and memory complexity of O(n)
+
+std::pair<int, int> solution(int sum, std::vector<int> v) {
+  std::unordered_map<int, int> indices_map;  // keeps array numbers as keys and
+                                             // indices as values behind keys
+
+  for (int i = 0; i < v.size(); i++) {
+    if (indices_map.find(sum - v[i]) !=
+        indices_map
+            .end()) {  // if key "number - t" exists, we have found the solution
+      return {indices_map[sum - v[i]], i};
+    }
+
+    if (indices_map.find(v[i]) == indices_map.end())
+      indices_map[v[i]] =
+          i;  // We only add keys that weren't in the map before (that
+              // way we get the least possible sum of i and j)
+  }
+
+  return {-1, -1};  // in case there are no such numbers
+}
\ No newline at end of file
diff --git a/task_01/src/test.cpp b/task_01/src/test.cpp
index ef5a86ae..2908b463 100644
--- a/task_01/src/test.cpp
+++ b/task_01/src/test.cpp
@@ -1,8 +1,39 @@
 
 #include <gtest/gtest.h>
 
-#include "topology_sort.hpp"
+#include "solution.h"
 
-TEST(TopologySort, Simple) {
-  ASSERT_EQ(1, 1);  // Stack []
-}
+TEST(solution, simple) {
+  std::vector<int> v1 = {-2, 2, 3, 3, 5, 9, 11, 13, 14, 15};
+  std::pair<int, int> p1 = {-1, -1};
+  ASSERT_EQ(p1, solution(10, v1));
+
+  std::vector<int> v2 = {-2, 2, 3, 3, 5, 8, 11, 13, 14, 15};
+  std::pair<int, int> p2 = {1, 5};
+  ASSERT_EQ(p2, solution(10, v2));
+
+  std::vector<int> v3 = {};
+  std::pair<int, int> p3 = {-1, -1};
+  ASSERT_EQ(p3, solution(0, v3));
+
+  std::vector<int> v4 = {1};
+  std::pair<int, int> p4 = {-1, -1};
+  ASSERT_EQ(p4, solution(1, v4));
+
+  std::vector<int> v5 = {1, 2};
+  std::pair<int, int> p5 = {0, 1};
+  ASSERT_EQ(p5, solution(3, v5));
+
+  // if there are multiple solutions, the algorithm
+  // will pick the one, in which sum of i and j is the least,
+  // where i and j are indices of numbers, which sum is equal to needed number
+  std::vector<int> v6 = {1, 1, 1, 1, 1, 1, 1, 1, 1};
+  std::pair<int, int> p6 = {0, 1};
+  ASSERT_EQ(p6, solution(2, v6));
+
+  // in case there are multiple solutions in which i+j is the least,
+  // the algorithm will pick the one, in which i is greater
+  std::vector<int> v7 = {1, 2, 1, 1, 4, 5, 1, 1, 1};
+  std::pair<int, int> p7 = {1, 4};
+  ASSERT_EQ(p7, solution(6, v7));
+}
\ No newline at end of file
diff --git a/task_01/src/topology_sort.cpp b/task_01/src/topology_sort.cpp
deleted file mode 100644
index e53f670c..00000000
--- a/task_01/src/topology_sort.cpp
+++ /dev/null
@@ -1 +0,0 @@
-#include "topology_sort.hpp"
diff --git a/task_01/src/topology_sort.hpp b/task_01/src/topology_sort.hpp
deleted file mode 100644
index 6f70f09b..00000000
--- a/task_01/src/topology_sort.hpp
+++ /dev/null
@@ -1 +0,0 @@
-#pragma once
diff --git a/testfile.txt b/testfile.txt
new file mode 100644
index 00000000..e69de29b