Min Stack

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : One result line per command.
 * Approach: A stack with a helper stack.
 * Time    : O(1) per command
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: One stack keeps the values. A second stack keeps the smallest value seen up to each level.
        static class MinStack {
            private final java.util.ArrayDeque<Long> values = new java.util.ArrayDeque<>();
            private final java.util.ArrayDeque<Long> minimums = new java.util.ArrayDeque<>();

            void push(long value) {
                values.push(value);
                // Step 2: The new minimum is the smaller of the new value and the old minimum.
                if (minimums.isEmpty() || value < minimums.peek()) {
                    minimums.push(value);
                } else {
                    minimums.push(minimums.peek());
                }
            }

            long pop() {
                minimums.pop();
                return values.pop();
            }

            long top() {
                return values.peek();
            }

            long min() {
                return minimums.peek();
            }

            boolean isEmpty() {
                return values.isEmpty();
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        MinStack stack = new MinStack();
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] parts = lines[i].split(" ");

            // Step 3: Run the command. Push always works. The other commands need a non-empty stack.
            if (parts[0].equals("push")) {
                stack.push(Long.parseLong(parts[1]));
                output[i] = "ok";
            } else if (stack.isEmpty()) {
                output[i] = "empty";
            } else if (parts[0].equals("pop")) {
                output[i] = String.valueOf(stack.pop());
            } else if (parts[0].equals("top")) {
                output[i] = String.valueOf(stack.top());
            } else {
                output[i] = String.valueOf(stack.min());
            }
        }
        return output;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(String.join("\n", solve(lines)));
    }
}