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12年前就关闭了。
有没有一种方法，我可以优化这个代码，以不运行内存不足？

import java.util.HashMap;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Random;
import java.util.Stack;

public class TilePuzzle {

    private final static byte ROWS = 4;
    private final static byte COLUMNS = 4;
    private static String SOLUTION = "123456789ABCDEF0";
    private static byte RADIX = 16;

    private char[][] board = new char[ROWS][COLUMNS];
    private byte x; // Row of the space ('0')
    private byte y; // Column of the space ('0') private String representation;
    private boolean change = false; // Has the board changed after the last call to toString?

    private TilePuzzle() {
        this(SOLUTION);
        int times = 1000;
        Random rnd = new Random();
        while(times-- > 0) {
            try {
                move((byte)rnd.nextInt(4));
            }
            catch(RuntimeException e) {
            }
        }
        this.representation = asString();
    }

    public TilePuzzle(String representation) {
        this.representation = representation;
        final byte SIZE = (byte)SOLUTION.length();
        if (representation.length() != SIZE) {
            throw new IllegalArgumentException("The board must have " + SIZE + "numbers.");
        }

        boolean[] used = new boolean[SIZE];
        byte idx = 0;
        for (byte i = 0; i < ROWS; ++i) {
            for (byte j = 0; j < COLUMNS; ++j) {
                char digit = representation.charAt(idx++);
                byte number = (byte)Character.digit(digit, RADIX);
                if (number < 0 || number >= SIZE) {
                    throw new IllegalArgumentException("The character " + digit + " is not valid.");
                } else if(used[number]) {
                    throw new IllegalArgumentException("The character " + digit + " is repeated.");
                }
                used[number] = true;
                board[i][j] = digit;
                if (digit == '0') {
                    x = i;
                    y = j;
                }
            }
        }
    }

    /**
     * Swap position of the space ('0') with the number that's up to it.
     */
    public void moveUp() {
        try {
            move((byte)(x - 1), y);
        } catch(IllegalArgumentException e) {
            throw new RuntimeException("Move prohibited " + e.getMessage());
        }
    }

    /**
     * Swap position of the space ('0') with the number that's down to it.
     */
    public void moveDown() {
        try {
            move((byte)(x + 1), y);
        } catch(IllegalArgumentException e) {
            throw new RuntimeException("Move prohibited " + e.getMessage());
        }
    }

    /**
     * Swap position of the space ('0') with the number that's left to it.
     */
    public void moveLeft() {
        try {
            move(x, (byte)(y - 1));
        } catch(IllegalArgumentException e) {
            throw new RuntimeException("Move prohibited " + e.getMessage());
        }
    }

    /**
     * Swap position of the space ('0') with the number that's right to it.
     */
    public void moveRight() {
        try {
            move(x, (byte)(y + 1));
        } catch(IllegalArgumentException e) {
            throw new RuntimeException("Move prohibited " + e.getMessage());
        }
    }

    private void move(byte movement) {
        switch(movement) {
        case 0: moveUp(); break;
        case 1: moveRight(); break;
        case 2: moveDown(); break;
        case 3: moveLeft(); break;
        }
    }

    private boolean areValidCoordinates(byte x, byte y) {
        return (x >= 0 && x < ROWS && y >= 0 && y < COLUMNS);
    }

    private void move(byte nx, byte ny) {
        if (!areValidCoordinates(nx, ny)) {
            throw new IllegalArgumentException("(" + nx + ", " + ny + ")");
        }
        board[x][y] = board[nx][ny];
        board[nx][ny] = '0';
        x = nx;
        y = ny;
        change = true;
    }

    public String printableString() {
        StringBuilder sb = new StringBuilder();
        for (byte i = 0; i < ROWS; ++i) {
            for (byte j = 0; j < COLUMNS; ++j) {
                sb.append(board[i][j] + " ");
            }
            sb.append("\r\n");
        }
        return sb.toString();
    }

    private String asString() {
        StringBuilder sb = new StringBuilder();
        for (byte i = 0; i < ROWS; ++i) {
            for (byte j = 0; j < COLUMNS; ++j) {
                sb.append(board[i][j]);
            }
        }
        return sb.toString();
    }

    public String toString() {
        if (change) {
            representation = asString();
        }
        return representation;
    }

    private static byte[] whereShouldItBe(char digit) {
        byte idx = (byte)SOLUTION.indexOf(digit);
        return new byte[] { (byte)(idx / ROWS), (byte)(idx % ROWS) };
    }

    private static byte manhattanDistance(byte x, byte y, byte x2, byte y2) {
        byte dx = (byte)Math.abs(x - x2);
        byte dy = (byte)Math.abs(y - y2);
        return (byte)(dx + dy);
    }

    private byte heuristic() {
        byte total = 0;
        for (byte i = 0; i < ROWS; ++i) {
            for (byte j = 0; j < COLUMNS; ++j) {
                char digit = board[i][j];
                byte[] coordenates = whereShouldItBe(digit);
                byte distance = manhattanDistance(i, j, coordenates[0], coordenates[1]);
                total += distance;
            }
        }
        return total;
    }

    private class Node implements Comparable<Node> {
        private String puzzle;
        private byte moves; // Number of moves from original configuration
        private byte value; // The value of the heuristic for this configuration.
        public Node(String puzzle, byte moves, byte value) {
            this.puzzle = puzzle;
            this.moves = moves;
            this.value = value;
        }
        @Override
        public int compareTo(Node o) {
            return (value + moves) - (o.value + o.moves);
        }
    }

    private void print(Map<String, String> antecessor) {
        Stack toPrint = new Stack();
        toPrint.add(SOLUTION);
        String before = antecessor.get(SOLUTION);
        while (!before.equals("")) {
            toPrint.add(before);
            before = antecessor.get(before);
        }
        while (!toPrint.isEmpty()) {
            System.out.println(new TilePuzzle(toPrint.pop()).printableString());
        }
    }

    private byte solve() {
        if(toString().equals(SOLUTION)) {
            return 0;
        }

        PriorityQueue<Node> toProcess = new PriorityQueue();
        Node initial = new Node(toString(), (byte)0, heuristic());
        toProcess.add(initial);

        Map<String, String> antecessor = new HashMap<String, String>();
        antecessor.put(toString(), "");

        while(!toProcess.isEmpty()) {
            Node actual = toProcess.poll();
            for (byte i = 0; i < 4; ++i) {
                TilePuzzle t = new TilePuzzle(actual.puzzle);
                try {
                    t.move(i);
                } catch(RuntimeException e) {
                    continue;
                }
                if (t.toString().equals(SOLUTION)) {
                    antecessor.put(SOLUTION, actual.puzzle);
                    print(antecessor);
                    return (byte)(actual.moves + 1);
                } else if (!antecessor.containsKey(t.toString())) {
                    byte v = t.heuristic();
                    Node neighbor = new Node(t.toString(), (byte)(actual.moves + 1), v);
                    toProcess.add(neighbor);
                    antecessor.put(t.toString(), actual.puzzle);
                }
            }
        }
        return -1;
    }

    public static void main(String... args) {
        TilePuzzle puzzle = new TilePuzzle();
        System.out.println(puzzle.solve());
    }
}