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List(CS61B学习记录)

news 2025/9/15 0:47:53

问题引入

上图中，赋给b海象的weight会改变a海象的weight，但x的赋值又不会改变y的赋值

Bits

要解释上图的问题，我们应该从Java的底层入手
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相同的二进制编码，却因为数据类型不同，输出不同的值

变量的声明

基本类型

Java does not write anything into the reserved box when a variable is declared. In other words, there are no default values. As a result, the Java compiler prevents you from using a variable until after the box has been filled with bits using the = operator. For this reason, I have avoided showing any bits in the boxes in the figure above. 在这里插入图片描述

引用类型

When we declare a variable of any reference type (Walrus, Dog, Planet, array, etc.), Java allocates a box of 64 bits, no matter what type of object.
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96位大于64位，这似乎有些矛盾：
在Java中：the 64 bit box contains not the data about the walrus, but instead the address of the Walrus in memory.

Gloden rule

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练习

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答案：B
解析：

在调用方法（函数）时，doStuff方法中的int x与main方法中的int x 实际上处于两个不同的scope（调用方法时会new 一个scope，并将main方法中的x变量的位都传递给doStuff方法中的x变量，即值传递），所以x = x - 5实际上只作用于doStuff方法中的x，而不是main方法中的x。
但对于引用类型来说，引用类型储存的是引用的地址，所以在进行值传递时传递的是对象的地址，所以doStuff方法中的int x与main方法中的walrus实际上指向相同的一个对象，这使得doStuff中执行的语句会作用于main方法中walrus指向的对象，所以反作用于main方法中的walrus

IntLists

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使用递归求链表中元素的个数

/** Return the size of the list using... recursion! */
public int size() {if (rest == null) {return 1;}return 1 + this.rest.size();
}

Exercise: You might wonder why we don’t do something like if (this == null) return 0;. Why wouldn’t this work?

Answer: Think about what happens when you call size. You are calling it on an object, for example L.size(). If L were null, then you would get a NullPointer error!

不使用递归求元素个数

/** Return the size of the list using no recursion! */
public int iterativeSize() {IntList p = this;int totalSize = 0;while (p != null) {totalSize += 1;p = p.rest;}return totalSize;
}

求第n个元素

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SLList

接下来我们对IntList进行改进，使链表看上去不那么naked
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有了节点类，现在我们补充上链表类

public class SLList {public IntNode first;public SLList(int x) {first = new IntNode(x, null);}
}

对比SLList和IntList，我们发现，在使用SLList时无需强调null

IntList L1 = new IntList(5, null);
SLList L2  = new SLList(5);

addFirst() and getFirst()

  /** Adds an item to the front of the list. */public void addFirst(int x) {first = new IntNode(x, first);}/** Retrieves the front item from the list. */public int getFirst() {return first.item;
}

private and nested class

public class SLList {public class IntNode {public int item;public IntNode next;public IntNode(int i, IntNode n) {item = i;next = n;}}private IntNode first;public SLList(int x) {first = new IntNode(x, null);}/** Adds an item to the front of the list. */public void addFirst(int x) {first = new IntNode(x,first);}/** Retrieves the front item from the list. */public int getFirst() {return first.item;}
}

addLast() and Size()

/** Adds an item to the end of the list. */
public void addLast(int x) {IntNode p = first;/* Advance p to the end of the list. */while (p.next != null) {p = p.next;}p.next = new IntNode(x, null);
}

/** Returns the size of the list starting at IntNode p. */
private static int size(IntNode p) {if (p.next == null) {return 1;}return 1 + size(p.next);
}

优化低效的Size()方法

在改变链表的Size时直接记录下，就可以不需要在Size()方法中遍历链表了

public class SLList {... /* IntNode declaration omitted. */private IntNode first;private int size;public SLList(int x) {first = new IntNode(x, null);size = 1;}public void addFirst(int x) {first = new IntNode(x, first);size += 1;}public int size() {return size;}...
}

空链表（ The Empty List)

创建空链表很简单（对于SLList），但会导致addLast()方法报错

public SLList() {first = null;size = 0;
}

public void addLast(int x) {size += 1;IntNode p = first;while (p.next != null) {p = p.next;}p.next = new IntNode(x, null);
}

p指向null，故p.next会出现空指针异常

addLast()改进（使用分支）

public void addLast(int x) {size += 1;if (first == null) {first = new IntNode(x, null);return;}IntNode p = first;while (p.next != null) {p = p.next;}p.next = new IntNode(x, null);
}

addLast()改进（更robust的做法）

对于存在很多特殊情况需要讨论的数据结构，上面的方法就显得十分低效。
故我们需要考虑将其改进为一个具有普适性的方法：将first改为sentinal.next
sentinal将会永远存在于链表的上位

package lists.sslist;public class SLList {public class IntNode {public int item;public IntNode next;public IntNode(int i, IntNode n) {item = i;next = n;}}private IntNode sentinel;private int size;public SLList() {sentinel = new IntNode(63,null);size = 0;}public SLList(int x) {sentinel = new IntNode(63,null);sentinel.next = new IntNode(x, null);size = 1;}/** Adds an item to the front of the list. */public void addFirst(int x) {sentinel.next = new IntNode(x, sentinel.next);size += 1;}/** Retrieves the front item from the list. */public int getFirst() {return sentinel.next.item;}/** Returns the number of items in the list. */public int size() {return size;}/** Adds an item to the end of the list. */public void addLast(int x) {IntNode p = sentinel;/* Advance p to the end of the list. */while (p.next != null) {p = p.next;}p.next = new IntNode(x, null);}/** Crashes when you call addLast on an empty SLList. Fix it. */public static void main(String[] args) {SLList x = new SLList();x.addLast(5);}
}