从“从头开始” Java链表中的Swap方法出现问题

Question

我已经对此进行了白板处理，似乎无法理解为什么出现内存不足错误。 该项目是从头开始创建一个链表及其一些方法。 我的其他功能很好，但是此交换功能给我带来了很多麻烦。

当我运行调试器时，程序在pj.nextNodeLink = p.nextNodeLink上崩溃。 swap函数应该接受两个int输入并交换它们的值。 我试图更改nextNodeLink指针来这样做，但显然失败了。 任何帮助将非常感激！

 public void swapByIndex(int firstIndexValue, int secondIndexValue){
    if(firstIndexValue<0 || secondIndexValue<0 || firstIndexValue>size-1 || secondIndexValue>size-1) {
        throw new ArrayIndexOutOfBoundsException();
    }
    else if(head == tail){ // Case one - only one element in list
        System.out.println("The list only has one element. Nothing to swap. ");
    }
    else{ // Case Two - two or more elements
        //keep a pointer to the next element of head
        Node firstPointer = head;
        Node firstSwapElement = firstPointer;
        for(int k=0; k<firstIndexValue; k++){
            firstSwapElement = firstPointer; // save the node P is on into 'previ' node
            firstPointer = firstPointer.nextNodeLink; // P iterates to next node
        }

        Node secondPointer = head;
        Node secondSwapElement = secondPointer;
        for(int k=0; k<secondIndexValue; k++){
            secondSwapElement = secondPointer;
            secondPointer = secondPointer.nextNodeLink;
        }

        Node secondNodeSave = secondPointer; // save this so we have the correct next node link for second swap

        secondPointer.nextNodeLink = firstPointer.nextNodeLink;
        firstSwapElement.nextNodeLink = secondSwapElement;
        firstPointer.nextNodeLink = secondNodeSave.nextNodeLink;
        secondSwapElement.nextNodeLink = secondPointer;

    }
}

Answer 1

代码的工作方式-您要交换的A和B节点：

Say initially you had, A - a - a2.... B - b- b2

b.next = a.next;  // b - a2
A.next = B;   // A - B
a.next = secondNodeSave.next; // a - b2
B.next = b;  // B - b

结合所有看起来像：

A - B - b - a2 - ?
a - b2 - ?

而预期的是：

B - a - a2.... A - b- b2

您的逻辑和变量放置完全为交换操作所困扰。 您无需关心a2或b2 ，它们在上下文中无关紧要。

正确的做法：

aPre - A - aNext ..... bPre - B - bNext

aPre->next = B
B->next    = aNext
bPre->next = A
A->next    = bNext

Answer 2

我刚刚编辑了您的代码。 只需运行它，它将解决您的问题。 您在交换节点时做错了。

 public void swapByIndex(int firstIndexValue, int secondIndexValue){
            if(firstIndexValue<0 || secondIndexValue<0 || firstIndexValue>size-1 || secondIndexValue>size-1) {
                throw new ArrayIndexOutOfBoundsException();
            }
            else if(head == tail){ // Case one - only one element in list
                System.out.println("The list only has one element. Nothing to swap. ");
            }else 
                 if(firstIndexValue==secondIndexValue)
                  {
                     System.out.print("Both are pointing to same index, no need to swap");
                  }
            else{ // Case Two - two or more elements
                //keep a pointer to the next element of head
                Node firstPointer = head;
                Node firstSwapElement = firstPointer;
                for(int k=0; k<firstIndexValue; k++){
                    firstSwapElement = firstPointer; // save the node P is on into 'previ' node
                    firstPointer = firstPointer.nextNodeLink; // P iterates to next node
                }

                Node secondPointer = head;
                Node secondSwapElement = secondPointer;
                for(int k=0; k<secondIndexValue; k++){
                    secondSwapElement = secondPointer;
                    secondPointer = secondPointer.nextNodeLink;
                }

                Node secondNodeSave = secondPointer; // save this so we have the correct next node link for second swap

                secondPointer.nextNodeLink = firstPointer.nextNodeLink;
                firstSwapElement.nextNodeLink = secondPointer;
                firstPointer.nextNodeLink = secondNodeSave.nextNodeLink;
                secondSwapElement.nextNodeLink = firstPointer;

            }
        }

Answer 3

最终使用了另一种策略-我没有实际尝试通过重新排列下一个指针来交换节点本身，而是简单地使用temp变量交换了值。

  public void swapByIndex(int firstIndexValue, int secondIndexValue){
    if(firstIndexValue<0 || secondIndexValue<0 || firstIndexValue>size-1 || secondIndexValue>size-1) {
        throw new ArrayIndexOutOfBoundsException();
    }
    else if(head == tail){ // Case one - only one element in list
        System.out.println("The list only has one element. Nothing to swap. ");
    }
    else{ // Case Two - two or more elements
        //keep a pointer to the next element of head
        Node firstSwapElement = head; //THIS IS THE FIRST ELEMENT!
        Node previousFirstSwapElement = firstSwapElement;
        for(int k=0; k<firstIndexValue; k++){
            previousFirstSwapElement = firstSwapElement; // save the node P is on into 'previ' node
            firstSwapElement = firstSwapElement.nextNodeLink; // P iterates to next node
        }

        Node secondSwapElement = head;
        Node previousSecondSwapElement = secondSwapElement;
        for(int k=0; k<secondIndexValue; k++){
            previousSecondSwapElement = secondSwapElement;
            secondSwapElement = secondSwapElement.nextNodeLink;
        }

        Integer temp = (Integer)secondSwapElement.data;
        secondSwapElement.data = firstSwapElement.data;
        firstSwapElement.data = temp;

    }
}