链表Java替换节点

Question

I'm having difficulty with a linked lists program. I want to write a method that destructively replaces the value in each node n by the sum of the values in the tail of the list. So if the list is 2,3,5,7; I want to change it to 17,15,12,7. I am given a program where I have to add a method that does this. I can change the first number, but I can not change the other three, and I am stuck. If anyone could help me, that would be great.

Original Program

public class IntList {
private int value;     
private IntList next;


public IntList(int v, IntList n) {          // Constructor
    value = v;
    next = n;
  }

public int getValue() { return value; }       // Getters
public IntList getNext() { return next; }
public void setValue(int v) { value = v; }    // Setters
public void setNext(IntList n) { next = n; }

// Find the last node of a linked list.
public IntList findLast() {
   if (getNext() == null) return this;
   else return getNext().findLast();
 }

// Add a new node with value v at the end of l;

public void addEnd(int v) {
    findLast().setNext(new IntList(v,null));
  }

// Add up the values in a list, recurring down the owner
public int sumList() {
   if (getNext() == null) return getValue();
   else return getValue() + getNext().sumList();
  }


// Convert list of int to string

// Recursive method for constructing the end of the string, after the
// initial open bracket.

 public String toString1() {
   if (getNext() == null)
      return getValue() + "]";
   else return getValue() + ", " + getNext().toString1();
 }

// Top level rountine that starts with the "[" and then calls toString1 to
// do the rest.

  public String toString() {
    return "[" + toString1();
  }

// Recursive method for finding the sum of a list, using recursion down
// an argument. Note that this is a static method, associated with the class
// not with an object owner.

 static public int sumListArg(IntList l) {
    if (l==null) return 0;
    else return l.getValue() + sumListArg(l.getNext());
   }

 static public void main(String[] args) {
   IntList l = new IntList(2,null);
   l.addEnd(3);
   l.addEnd(5);
   l.addEnd(7);
   System.out.println("h");
   System.out.println(l.toString());
   System.out.println("Sum = " + l.sumList());
} // end main
} // end RecursiveIntList    

Here is what I have so far for my method (I think it's logically ok, but it's incorrect):

 public static void runningSum(IntList l)
{ 
     l.setValue(l.sumList());

    while(l.getNext() != null) 
     {
        l.setNext(l.getNext()); //Set Next to be the next reference
        l.getValue();  //Get the Next's value
        l.setValue(l.sumList()); //Add the rest of the numbers together
     }

     if(l.getNext() == null)
     {
         l.setValue(l.getValue());
     }

     System.out.println(l.toString());
}

Answer 1

There is a nice and elegant solution:

public int sumList() {
    if (getNext() == null) {
        return getValue();
    }
    value = getValue() + getNext().sumList();
    return value;
 }

In this method you recursively iterate through the List, and summarize all the elements that are behind the current element, and set the value at the same time.

Answer 2

This is the code:

public static void runningSum(IntList l)
{ 
    IntList head = l;
    int rSum = l.sumList();

    while(l != null) 
    {
        int curRS = rSum;
        curRS -= l.getValue();
        l.setValue(rSum);
        rSum = curRS;
        l = l.getNext();
    }

    System.out.println(head.toString());
}

I'll break it in several parts to explain what's going on. We want to code a procedure that takes the head of a list and alters the list in the way you describe; basically the first element must become the sum of all the original elements; the second element must be the sum of all elements except the first, and so on. The last element, the tail, must remain unchanged.

public static void runningSum(IntList l)
{

The function we need to remember the head that was passed to the function; we save l in a variable called head .

    IntList head = l;

The running sum for the first element is the sum of all elements; so we call sumList and store the result in a variable called rSum .

    int rSum = l.sumList();

This is a very typical idiom in data structures programming; while the element is not null, you loop.

    while(l != null) 
    {

The running sum of the next element is rSum minus the value of the current element.

        int nextRS = rSum - l.getValue();

Now we can set running sum of the current element to rSum .

        l.setValue(rSum);

For the next iteration, the current running sum is nextRS . Finally we update l to point to the next element.

        rSum = nextRS;
        l = l.getNext();
    }

If we didn't keep track of head , now we wouldn't know what to print.

    System.out.println(head.toString());
}

Answer 3

Linked lists lend themselves to recursion. A recursive solution might look like this:

public static void runningSum(IntList l) {
    if (l.getNext() != null) {
        runningSum(l.getNext());
        l.setValue(l.getValue() + l.getNext().getValue());
    }
}