從頭開始排序插入單鏈接列表

Question

我一直在嘗試從頭開始創建僅包含字符串，但在插入時對其進行排序的排序鏈表。 這是我目前的代碼：

import java.util.Arrays;

public class SortedLinkedList {

    private StringNode head = null;

    /**
     * Default Constructor for a sorted linked list
     */
    public SortedLinkedList() {}

    /**
     * Will add a new node with the specified data to the correctly sorted 
     * position in the list
     */
    public void add(String data) {

        if (head == null) {
            head = new StringNode(data);
        }

        StringNode temp = new StringNode(data);
        StringNode current = head;

        if (current != null) {
            while (current.getNext() != null) {
                if (current.getData().toString().compareTo(current.getNext().getData().toString()) < 0) {
                    temp.setNext(current.getNext());
                    current.setNext(temp);
                } else
                    current.setNext(temp);
            }
        }
    }

    /**
     * Will remove the node that matches the specified data from the list.
     * Returns true if node is found, otherwise will return false
     */
    public boolean remove(String data) {
        StringNode current = head;

        if (head != null) {

            while (current.getNext() != null) {

                if (current.getData().toString().equals(data)) {
                    current.setNext(current.getNext().getNext());
                    return true;
                }

                current = current.getNext();
            }
        }
        return false;
    }

    /**
     * Will cycle through the list and display each item on a new line
     */
    public void display() {

        if (head != null) {

            StringNode current = head.getNext();
            System.out.print("[");

            while (current.getNext() != null) {
                System.out.print(current.getData().toString() + ", ");
                current = current.getNext();
            }

            System.out.print("]");
        }
    }

    // Inner Class 

    class StringNode {

        String data;
        StringNode next;

        public StringNode(String nodeData) {
            next = null;
            data = nodeData;
        }

        /**
         * Getter of Data
         */
        public String getData() {
            return data;
        }

        /**
         * Getter of Next
         */
        public StringNode getNext() {
            return next;
        }

        /**
         * Setter of Data
         */
        public void setData(String newData) {
            data = newData;
        }

        /**
         * Setter of Next
         */
        public void setNext(StringNode nextNode) {
            next = nextNode;
        }
    }

}

我已經能夠添加而無需插入排序，但是，當我嘗試在插入編碼中編寫代碼后，它就壞了。 似乎它不再添加任何值。 我的驅動程序的當前輸出：

public class Driver {
    public static void main(String[] args) {
        SortedLinkedList name = new SortedLinkedList();

        name.add("v");
        name.add("a");
        name.add("b");

        name.display();
    }
}

在我的顯示方法第70行是一個空指針異常，這是我的while循環的創建。 我完全迷失了，需要一些指導。 謝謝。

Answer 1

實際上你想檢查current == null，因為最后一個元素的.getNext（）是null。

while (current != null)

重構一點：

// should print [] for empty SLL; no dangling comma
public void display() {

  String out = "[";
  StringNode current = head;

  while(current != null) {
      out = out + current.getData();
      current = current.getNext();
      if (current != null)
        out = out + ", ";
  }
  out += "]";
  System.out.print(out);
}

編輯：此外，您的添加/刪除方法需要重新設計...嘗試在紙上逐步研究算法，然后將條件/操作轉換為Java。

例：

/**
 * Will add a new node with the specified data to the correctly sorted 
 * position in the list
 */
public void add(String data) {
  StringNode temp = new StringNode(data);

  if(head == null) {
      head = temp;
      return; // only adding one node
  }

  StringNode previous = head;

  if (data.compareTo(previous.getData()) < 0) {
     head = temp; // temp < head, so we add it to the beginning
     head.setNext(previous);
     return; // done
  }

  StringNode current = previous.getNext();

  while (current != null) {
      if (data.compareTo(current.getData()) < 0) {
          temp.setNext(current);
          previous.setNext(temp);
          return; // done
      } else {
          previous = current;
          current = previous.getNext();
      }
  }
  // current == null, so we reached the end of the list;
  previous.setNext(temp);
}

Answer 2

我稍微重構了你的代碼並刪除了你寫的代碼注釋。 相反，我添加了一些自己的評論。

請仔細閱讀這些注釋，並考慮為您編寫的任何代碼編寫單元測試。 它將幫助您捕獲任何錯誤，並讓您對發送的代碼充滿信心。

package com.so36046948;

public class SortedLinkedList {

  private StringNode head = null;

  public SortedLinkedList() {}

  public void add(String data) {
    // Do null checks on Data.
    if (head == null) {
      // If we have no head, set head and return immediately.
      head = new StringNode(data, null);
      return;
    }

    StringNode previous = head;
    StringNode current = head;
    StringNode temp = new StringNode(data);

    // Continue iterating till we reach the end of the list.
    while (null != current) {

      if (current.getData().compareTo(data) < 0) {
        if (current == head) {
          // Special handling for the case when the element being inserted is greater than head.
          head = temp;
          temp.setNext(current);
          return;
        }
        break;
      }
      previous = current;
      current = current.getNext();
    }

    // Break out of the while loop and reset the next pointers. Previous points to immediate 
    // greater element and current points to immediate element that is less than *data* being
    // inserted.
    previous.setNext(temp);
    temp.setNext(current);
  }

  public void display() {
    // Consider overriding toString method instead of creating a display method,
    if (null == head) {
      System.out.println("List is empty.");
      return;
    }

    StringBuilder sb = new StringBuilder();
    StringNode current = head;
    sb.append("[");
    while (current != null) {
      sb.append(current.getData()).append(",");
      current = current.getNext();
    }
    sb.append("]");
    System.out.println(sb);
  }

  // Inner Class

  class StringNode {

    private final String data;
    private StringNode next;

    public StringNode(String nodeData) {
      next = null;
      data = nodeData;
    }

    public StringNode(String nodeData, StringNode next) {
      this.next = next;
      data = nodeData;
    }

    /**
     * Getter of Data
     */
    public String getData() {
      return data;
    }

    /**
     * Getter of Next
     */
    public StringNode getNext() {
      return next;
    }

    /**
     * Setter of Next
     */
    public void setNext(StringNode nextNode) {
      next = nextNode;
    }
  }

  public static void main(String[] args) {
    // Consider writing unit tests. I have created skeletons of a few rudimentry tests,
    // you may add more.
    test_insertInDscendingOrder();
    test_insertInAscendingOrder();
    test_insertTwoElements();
    test_insertSingleElement();
    //test_insertNullData();
  }

  private static void test_insertInDscendingOrder() {
    SortedLinkedList name = new SortedLinkedList();
    name.add("v");name.add("b");name.add("a");name.display();
  }

  private static void test_insertInAscendingOrder() {
    SortedLinkedList name = new SortedLinkedList();
    name.add("a");name.add("b");name.add("c");name.add("d");
    name.display();
  }

  private static void test_insertTwoElements() {
    SortedLinkedList name = new SortedLinkedList();
    name.add("a");name.add("b");name.display();
  }

  private static void test_insertSingleElement() {
    SortedLinkedList name = new SortedLinkedList();
    name.add("a");name.display();
  }

  private static void test_insertNullData() {
    // This would fail, Consider handling null data.
    SortedLinkedList name = new SortedLinkedList();
    name.add("a");
    name.add(null);
    name.display();
  }
}