Expression Tokenizer

/*
 * Problem : Split into tokens.
 * Input   : One expression.
 * Output  : One token per line.
 * Approach: Character scanner.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String s) {
        // Step 1: Scan the characters from left to right. A digit starts a number: read all the digits that follow.
        java.util.List<String> tokens = new java.util.ArrayList<String>();
        int i = 0;
        while (i < s.length()) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                int start = i;
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    i++;
                }
                tokens.add(s.substring(start, i));
            } else {
                // Step 2: Spaces are skipped. Every other character is a one-character token.
                if (c != ' ') {
                    tokens.add(String.valueOf(c));
                }
                i++;
            }
        }
        return tokens.toArray(new String[0]);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(String.join("\n", solve(s)));
    }
}