使用節點類乘以多項式

Question

我已經創建了一個數據結構以使用本地節點類來排斥多項式。我有方法來加減兩個長度爲n的多項式。我試圖添加一個乘法方法，它將返回兩個多項式的乘積。我幾乎完成了這個問題。在這一點上，我試圖創建第三個鏈表（使用Node類）來存儲和返回兩個多項式的乘積。我的問題只是編碼這個解決方案。我得到空指針異常。

這裏是我的節點類完整代碼：

public class Node{ 

//Fields 
public int data1; 
public int data2; 
public Node next; 



//constructors 
public Node(){ 
    data1 = 0; 
    data2 = 0; 
    next = null; 
} 


public Node(int d1){ 
    data1 = d1; 
    data2 = 0; 
    next = null; 
} 

public Node(int d1, Node n){ 
    data1 = d1; 
    data2 = 0; 
    next = n; 
} 

public Node(int d1, int d2){ 
    data1 = d1; 
    data2 = d2; 
    next = null; 
} 
public Node(int d1,int d2, Node n){ 
    data1 = d1; 
    data2 = d2; 
    next = n; 
} 

//Methods 

//Fetch data 
public int getData1(){ 
    return data1; 
} 

public int getData2(){ 
    return data2; 
} 

//store Data 
public void setData1(int d){ 
    data1 = d; 
} 

public void setData2(int d){ 
    data2 = d; 
} 

public int addData1(Node n2){ 
    data1 = data1 + n2.data1; 
    return data1; 
} 

public int addData2(Node n2){ 
    data2 = data2 + n2.data2; 
    return data2; 
} 

public void subData1(Node n2){ 
    data1 = data1 - n2.data1; 
} 

public int multiplyData1(Node n2){ 
    data1 = data1*n2.data1; 
    return data1; 
} 

//getNext 
public Node getNext(){ 
    return next; 
} 

//Get data of next node 
public int getNextData1(){ 
    return next.data1; 
} 

public int getNextData2(){ 
    return next.data2; 
} 

//Store Link 
public void setNext(Node n){ 
    next = n; 
} 

public boolean containsLink(){ 
    if(this.next != null){ 
     return true; 
    }else{ 
     return false; 
    } 
} 

public static void displayAll(Node head){ 
    for(; head != null; head = head.next){ 
     System.out.printf("%d, %d\n", head.data1, head.data2); 
    } 
} 

public static int numOfNonZeroData1(Node n){ 
    int count = 0; 
    for(; n != null ; n = n.next){ 
     if(n.data1 != 0){ 
      count++; 
     } 
    } 
    return count; 
} 

public static int numOfNonZeroData2(Node n){ 
    int count = 0; 
    for(; n != null ; n = n.next){ 
     if(n.data2 != 0){ 
      count++; 
     } 
    } 
    return count; 
} 

public static int listLength(Node head){ 
    int counter = 0; 
    for(; head!=null; head = head.next){ 
     counter++; 
    } 
    return counter; 
} 

//copy list [Recursive method found on website StackOverflow.com] 
public Node copyData1(Node p) { 
    if(p == null) 
     return null; 
    else 
     return new Node(p.data1, copyData1(p.next)); 
} 

public Node copyData2(Node p) { 
    if(p == null) 
     return null; 
    else 
     return new Node(p.data2, copyData2(p.next)); 
} 

//=============================================================== 


public static void toPolynomial(Node head){ 
    int order = Node.listLength(head); 
    int i = 0; 
    int increment = Node.numOfNonZeroData2(head); 
    for(; head != null; head = head.next){ 
     if(head.data2 != 0 && head.data1 != 0){ 
      if(i >= 0 && i < order){ 
       System.out.printf("%dx^%d", head.data1, head.data2); 
       i++; 
       if(i < increment && head.data1 >= 0){ 
        System.out.print("+");  //case integer is positive 
       }else if(i != increment && head.data1 <= 0){ 
        System.out.println(" "); //case integer is negative 
       } 
      } 

     } 
    } 
    System.out.println(); 
} 




public static Node mergeLists(Node n1, Node n2){ 
    if (n1 == null) 
     return n2; 
    else if (n2 == null) 
     return n1; 
    else { 
     n1.next = mergeLists(n1.next, n2); 
     return n1; 
    } 
} 


public static Node addPolynomials(Node n1, Node n2) { 
    Node x = n1; 
    Node y = n2; 
    for(x = n1; x != null; x = x.next){ 
     for(y = n2; y != null; y = y.next){ 
      if(x.getData2() == y.getData2()){ 
       x.addData1(y); 
       System.out.println("Added " + (x.data1 - y.data1) + " and " + y.data1); 
      } 

     } 
    } 
    System.out.println("Add completed"); 
    return x; 
} 


public static Node subtractPolynomials(Node n1, Node n2){ 
    Node x = n1; 
    Node y = n2; 
    for(x = n1; x != null; x = x.next){ 
     for(y = n2; y != null; y = y.next){ 
      if(x.getData2() == y.getData2()){ 
       x.subData1(y); 
       System.out.println("Subtracted " + (x.data1 - y.data1) + " and " + y.data1); 
      } 

     } 
    } 
    System.out.println("Subtract completed"); 
    return x; 
} 


public static Node multiplyPolynomials(Node n1, Node n2){ 

    Node x = n1; 
    Node y = n2; 
    Node z = new Node(); 
    for(x = n1; x != null; x = x.next){ 
     for(y = n2; y != null; y = y.next){ 
      z.data1 = x.multiplyData1(y); // error is here 
      z.data2 = x.addData2(y); 
      //System.out.println("Multiplied " + (x.data1 - y.data1) + " and " + y.data1); 
      z = z.next; 
     } 
    } 
    System.out.println("Multiplication completed"); 
    return x; 
} 

}

什麼想法？

Answer 1

請看本節：

**Node z = new Node();** 
for(x = n1; x != null; x = x.next){ 
    for(y = n2; y != null; y = y.next){ 
     z.data1 = x.multiplyData1(y); 
     z.data2 = x.addData2(y); 
     //System.out.println("Multiplied " + (x.data1 - y.data1) + " and " + y.data1); 
     **z = z.next;** 
    } 
} 

您遍歷任意長度的節點列表，但新的z節點的長度爲1。你必須添加節點到Z，你附和。例如：

z.next = new Node(); 
z = z.next;