// Package collection implements a generic stack.
package collection

// The zero value for Stack is an empty stack ready to use.
type Stack struct {
    data []interface{}
}

// Push adds x to the top of the stack.
func (s *Stack) Push(x interface{}) {
    s.data = append(s.data, x)
}

// Pop removes and returns the top element of the stack.
// It’s a run-time error to call Pop on an empty stack.
func (s *Stack) Pop() interface{} {
    i := len(s.data) - 1
    res := s.data[i]
    s.data[i] = nil  // to avoid memory leak
    s.data = s.data[:i]
    return res
}

// Size returns the number of elements in the stack.
func (s *Stack) Size() int {
    return len(s.data)
}