Sparse Vector Class

/*
 * Problem : Run the commands.
 * Input   : Command lines.
 * Output  : One line per answer.
 * Approach: Sparse vector class
 * Time    : O(n x nnz)
 * Space   : O(nnz)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: Only the nonzero entries are stored, in a sorted map from index to value.
        static class SparseVector {
            private final java.util.TreeMap<Integer, Long> entries = new java.util.TreeMap<Integer, Long>();

            void set(int index, long value) {
                if (value == 0) {
                    entries.remove(index);
                } else {
                    entries.put(index, value);
                }
            }

            // Step 2: The dot product only needs indexes that exist in both vectors. Loop over the smaller map.
            long dot(SparseVector other) {
                long total = 0;
                for (java.util.Map.Entry<Integer, Long> e : entries.entrySet()) {
                    Long match = other.entries.get(e.getKey());
                    if (match != null) {
                        total += e.getValue() * match;
                    }
                }
                return total;
            }

            // Step 3: The sum starts as a copy of this vector, then the other entries are added. Zero results are dropped by set.
            SparseVector plus(SparseVector other) {
                SparseVector result = new SparseVector();
                for (java.util.Map.Entry<Integer, Long> e : entries.entrySet()) {
                    result.set(e.getKey(), e.getValue());
                }
                for (java.util.Map.Entry<Integer, Long> e : other.entries.entrySet()) {
                    long old = result.entries.containsKey(e.getKey()) ? result.entries.get(e.getKey()) : 0;
                    result.set(e.getKey(), old + e.getValue());
                }
                return result;
            }

            String text() {
                StringBuilder sb = new StringBuilder();
                for (java.util.Map.Entry<Integer, Long> e : entries.entrySet()) {
                    sb.append(sb.length() == 0 ? "" : " ").append(e.getKey()).append(":").append(e.getValue());
                }
                return sb.length() == 0 ? "empty" : sb.toString();
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        java.util.Map<String, SparseVector> named = new java.util.HashMap<String, SparseVector>();
        java.util.List<String> out = new java.util.ArrayList<String>();
        for (String line : lines) {
            String[] part = line.split(" ");
            if (part[0].equals("vec")) {
                SparseVector v = new SparseVector();
                for (int i = 2; i < part.length; i++) {
                    String[] pair = part[i].split(":");
                    v.set(Integer.parseInt(pair[0]), Long.parseLong(pair[1]));
                }
                named.put(part[1], v);
            } else if (part[0].equals("dot")) {
                out.add(String.valueOf(named.get(part[1]).dot(named.get(part[2]))));
            } else if (part[0].equals("add")) {
                named.put(part[1], named.get(part[2]).plus(named.get(part[3])));
            } else if (part[0].equals("norm2")) {
                out.add(String.valueOf(named.get(part[1]).dot(named.get(part[1]))));
            } else {
                out.add(named.get(part[1]).text());
            }
        }
        return out.toArray(new String[0]);
    }

    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)));
    }
}