chore: import upstream snapshot with attribution

data_structures/stack_using_linked_list.cpp

@@ -0,0 +1,66 @@
+#include <iostream>
+
+struct node {
+    int val;
+    node *next;
+};
+
+node *top_var;
+
+void push(int x) {
+    node *n = new node;
+    n->val = x;
+    n->next = top_var;
+    top_var = n;
+}
+
+void pop() {
+    if (top_var == nullptr) {
+        std::cout << "\nUnderflow";
+    } else {
+        node *t = top_var;
+        std::cout << "\n" << t->val << " deleted";
+        top_var = top_var->next;
+        delete t;
+    }
+}
+
+void show() {
+    node *t = top_var;
+    while (t != nullptr) {
+        std::cout << t->val << "\n";
+        t = t->next;
+    }
+}
+
+int main() {
+    int ch = 0, x = 0;
+    do {
+        std::cout << "\n0. Exit or Ctrl+C";
+        std::cout << "\n1. Push";
+        std::cout << "\n2. Pop";
+        std::cout << "\n3. Print";
+        std::cout << "\nEnter Your Choice: ";
+        std::cin >> ch;
+        switch (ch) {
+            case 0:
+                break;
+            case 1:
+                std::cout << "\nInsert : ";
+                std::cin >> x;
+                push(x);
+                break;
+            case 2:
+                pop();
+                break;
+            case 3:
+                show();
+                break;
+            default:
+                std::cout << "Invalid option!\n";
+                break;
+        }
+    } while (ch != 0);
+
+    return 0;
+}