来自递归方法的StackoverflowError

Question

我的作业需要帮助。 作业是编写递归方法，解决每个简单的级别 来自游戏“推箱子”。它应该使用Cloneable接口和hashSets来完成。 这就是我问这个的原因。我已经在我的代码中编写了递归方法，但它总是让我得到一个StackoverflowError。

我希望你能帮助我！

import java.util.HashSet;

public class Solver implements Cloneable {
    HashSet<Level> fieldSet;
    public Solver () {
        fieldSet = new HashSet<Level>();
    }
        public final static char WALL = '#';                                
        public final static char PLAYER = '@';
        public final static char GOAL = '.';
        public final static char CHEST = '$';
        public final static char CHESTONGOAL = '*';
        public final static char PLAYERONGOAL = '+';
        public final static char FLOOR = ' ';

    public String solve (Level l) {
        Level fieldUp = l.clone();
        Player playerUp = fieldUp.getPlayer();
        playerUp.move(0, -1);
        if (this.fieldSet.add(fieldUp)) {
            if (!fieldUp.isCompleted()) {
                return "up, " + this.solve(fieldUp);
            } else {
                return "up.";
            }
        }

        Level fieldRight = l.clone();
        Player playerRight = fieldRight.getPlayer();
        playerRight.move(1, 0);
        if (this.fieldSet.add(fieldRight)) {
            if (!fieldRight.isCompleted()) {
                return "right, " + this.solve(fieldRight);
            } else {
                return "right.";
            }
        }
        Level fieldDown = l.clone();
        Player playerDown = fieldDown.getPlayer();
        playerDown.move(0, 1);
        if (this.fieldSet.add(fieldDown)) {
            if (!fieldDown.isCompleted()) {
                return "down, " + this.solve(fieldDown);
            } else {
                return "down.";
            }
        }
        Level fieldLeft = l.clone();
        Player playerLeft = fieldLeft.getPlayer();
        playerLeft.move(-1, 0);
        if (this.fieldSet.add(fieldLeft)) {
            if (!fieldLeft.isCompleted()) {
                return "left, " + this.solve(fieldLeft);
            } else {
                return "left.";
            }
        }
        return "";
    }
}