有人能帮我找出我的代码有什么问题吗？当我运行它时，只显示第一个输入数字（它不创建排序列表），delete命令不起作用，exists命令中的'true''false'不显示。我的输出应该与我在最后给出的样本相匹配。
为了使代码正常工作，我必须填写的区域是todododo符号后面的区域，即44-61、75-83、97-105。我不知道我在这些方面哪里出了问题，为什么它不能正常工作，以提供所需的输出？

import java.util.Scanner;

// Defines the a Sorted Set collection and implements a driver program in main
public class SortedSet {
    // Define a basic element of a linked list
    private class LinkedNode {
        int x;  // Value stored in the node
        LinkedNode next;  // Reference to the next node in the list
    }

LinkedNode front = null;  // Reference to the front of the singly linked list

// Adds the integer x to the collection.
// The resulting collection is sorted in increasing order and
// does not contain any duplicate values.
public void add(int x) {
    // Initialize a new node to be added to the collection
    LinkedNode newNode = new LinkedNode();
    LinkedNode cur = front;
    newNode.x = x;

    // Check if list is empty
    if (cur == null) {
        front = newNode;
    } 
    // If list is not empty, check if node should be placed in front
    else if (front != null) {
        if (newNode.x < front.x) {
            newNode.next = front;
            front = newNode;
        }

        // If not in front, check for the middle or the end, or duplicate.
        else {
            // <TODO><TODO><TODO> 
            LinkedNode temp = cur;
            LinkedNode prev = cur;
            int middle = x;
            while (temp != null) {
                if(temp.x > newNode.x) {
                    middle = 1;
                    newNode.next = temp;
                    prev.next = newNode;
                }
                prev = temp;
                temp = temp.next;
                }
            if (middle == 0) {
                prev = newNode;
                }
            }
        }
    }

// Deletes the integer x from the sorted set.
// The remaining collection remains sorted and without duplicates.
public void delete(int x){
    // Declare a new reference and initialize it to the front of the list
    LinkedNode cur = front;

    // Check if list is empty
    if (front == null) {
        System.out.print("There is nothing to delete!");
    } else {  // Not empty
        // Go through list, checking whether node is in the list, and delete if found
        // <TODO><TODO><TODO>
        LinkedNode prev = new LinkedNode();
        while (cur.x != x && cur != null) {
            prev = cur;
            cur = cur.next;
            }
        if (cur != null)
            prev.next = cur.next;
        }
    }

// Returns true if the integer x exists in the sorted set and false otherwise.
public void exists(int x) {
    // Declare a new reference and initialize it to the front of the list
    LinkedNode cur = front;

    // Check if list is empty.
    if (front == null) {
        System.out.println("false");
    }

    // If not empty, check for the node.
    // <TODO><TODO><TODO>
    else {
        while (cur != null) {
            if (cur.x==x)
                return;
            cur=cur.next;
            }
        return;
    }
}

// Returns a string representing the sorted set as a space separated list.
public String toString() {
    String s = "";

    LinkedNode cur = front;     
    while (cur!=null) {
        s+= cur.x + " ";
        cur = cur.next;
    }

    return s;
}

// Driver method
public static void main(String[] args) {
    // Declare variables
    SortedSet sortedSet = new SortedSet();
    Scanner scan = new Scanner(System.in);
    String[] tokens;
    String command;
    int num;

    // Print header info
    System.out.println("Programming Fundamentals\n"
            + "NAME: Andres Reyes\n"
            + "PROGRAMMING ASSIGNMENT 4\n");

    // Enter command loop
    while (true) {
        // Prompt the user for a command
        System.out.print("Enter command: ");
        String input = scan.nextLine();

        // Parse input
        if (input.equals("q")) break;  // user quits
        tokens = input.split("\\s");
        if (tokens.length < 2) continue; // invalid input
        command = tokens[0];
        num = Integer.parseInt(tokens[1]);

        // Execute command
        if (command.equals("add")){
            sortedSet.add(num);
            System.out.println(sortedSet);
        } else if (command.equals("del")) {
            sortedSet.delete(num);
            System.out.println(sortedSet);
        } else if (command.equals("exists")) {
            sortedSet.exists(num);
        } else {
            System.out.print("Command does not exist");
        }
    }

    System.out.println("\nGood bye!");
}

}