Description : size() function is used to return the size of the satck or the number of elements in the stack.
Example:
// Empty stack
std::stack<int> mystack;
mystack.push(4);
mystack.push(5);
mystack.push(9);
mystack.push(1);
mystack.push(3);
// Stack from top to bottom becomes 3, 1, 9, 5, 4
// Print the size of mystack (5)
std::cout << mystack.size();