diff --git a/6_find_all_missing_numbers.cpp b/6_find_all_missing_numbers.cpp
new file mode 100644
index 0000000..a3e44de
--- /dev/null
+++ b/6_find_all_missing_numbers.cpp
@@ -0,0 +1,33 @@
+/* Problem:
+ * https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/
+ */
+
+#include <iostream>
+#include <ostream>
+#include <vector>
+
+std::ostream &operator<<(std::ostream &out, const std::vector<int> &&nums) {
+  out << "{";
+  for (const int num : nums)
+    out << num << ", ";
+  out << "}";
+  return out;
+}
+
+std::vector<int> solution(const std::vector<int> &input) {
+  std::vector<bool> num_exist(input.size(), false);
+  std::vector<int> missing_nums;
+  for (const int num : input)
+    num_exist[num - 1] = true;
+  for (int i = 0, upto = input.size(); i < upto; i++) {
+    if (!num_exist[i])
+      missing_nums.push_back(i + 1);
+  }
+  return missing_nums;
+}
+
+int main() {
+  std::cout << solution({4, 3, 2, 7, 8, 2, 3, 1}) << '\n'
+            << solution({1, 1}) << '\n';
+  return 0;
+}
\ No newline at end of file
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3c7fde9..bffcb39 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,3 +12,5 @@ add_executable(1_longest_substring_with_k_distinct_characters 1_longest_substrin
 add_executable(2_num_of_islands 2_num_of_islands.cpp)
 
 add_executable(3_squares_of_a_sorted_array 3_squares_of_a_sorted_array.cpp)
+
+add_executable(6_find_all_missing_numbers 6_find_all_missing_numbers.cpp)