我需要为大学项目实现Trie(用Java)。 Trie应该能够添加和删除字符串(第1阶段)。
我每天花费几个小时(最近几天)试图找出如何做到这一点并且每次都惨遭失败。
我需要一些帮助,互联网上的例子和我的教科书(Java中的数据结构和算法,Adam Drozdek)没有帮助。
我正在使用的节点类:
class Node {
public boolean isLeaf;
}
class internalNode extends Node {
public String letters; //letter[0] = '$' always.
//See image -> if letter[1] = 'A' then children[1] refers to child node "AMMO"
//See image -> if letter[2] = 'B' then children[2] refers to internal node "#EU"
public TrieNode[] children = new TrieNode[2];
public TrieInternalNode(char ch)
{
letters = "#" + String.valueOf(ch);//letter[0] = '$' always.
isLeaf = false;
}
}
class leafNode extends Node
{
public String word;
public TrieLeafNode(String word)
{
this.word = new String(word);
isLeaf = true;
}
}
以下是我需要遵循的插入伪代码:(警告它非常模糊)
trieInsert(String K)
{
i = 0;
p = the root;
while (not inserted)
{
if the end of word k is reached
set the end-of-word marker in p to true;
else if (p.ptrs[K[i]] == 0)
create a leaf containing K and put its address in p.ptrs[K[i]];
else if reference p.ptrs[K[i]] refers to a leaf
{
K_L = key in leaf p.ptrs[K[i]]
do
{
create a nonleaf and put its address in p.ptrs[K[i]];
p = the new nonleaf;
} while (K[i] == K_L[i++]);
}
create a leaf containing K and put its address in p.ptrs[K[--i]];
if the end of word k is reached
set the end-of-word marker in p to true;
else
create a leaf containing K_L and put its address in p.ptrs[K_L[i]];
else
p = p.ptrs[K[i++]];
}
}
我需要实现以下方法。
public boolean add(String word){...}//adds word to trie structure should return true if successful and false otherwise
public boolean remove(String word){...}//removes word from trie structure should return true if successful and false otherwise
我找不到要删除的伪代码,但如果插入不起作用删除不会帮助我。
以下是我需要实施的Trie的图片。
我知道如果像这样实施,Trie仍然效率低下,但目前我不用担心这个。
本书提供的实现类似于我需要做的但不使用单词char('$')的结尾,只存储没有前缀的单词在子节点{{1 }}
http://mathcs.duq.edu/drozdek/DSinJava/SpellCheck.java
(美元)符号来表示单词结尾。 (见下图)
$
符号的单词。我不希望任何人为我做这个实施(除非他们有一个躺着:P)我真的需要帮助。
答案 0 :(得分:2)
首先,我认为你不应该让叶子节点和内部节点分开类。我建议使用isLeaf()方法创建通用节点类。如果节点没有子节点,则此方法将返回true。
这是您需要实现的功能的一些更高级别的伪代码。为简单起见,我假设存在一个名为getIndex()的方法,该方法返回与字符对应的索引。
Insert(String str)
Node current = null
for each character in str
int index = getIndex(character)
if current.children[index] has not been initialized
initialize current.children[index] to be a new Node
current = current.children[index]
您可以轻松扩充此伪代码以满足您的需求。例如,如果要在插入不成功时返回false:
现在,这里有一些更高级别的伪代码用于删除。
Remove(String str)
Node current = null
for each character in str
int index = getIndex(character)
current = current.children[index]
// At this point, we found the node we want to remove. However, we want to
// delete as many ancestor nodes as possible. We can delete an ancestor node
// if it is not need it any more. That is, we can delete an ancestor node
// if it has exactly one child.
Node ancestor = current
while ancestor is not null
if ancestor has 2 or more children
break out of loop
else if ancestor has less than 2 children
Node grandAncestor = ancestor.parent
if grandAncestor is not null
reinitialize grandAncestor.children // this has the effect of removing ancestor
ancestor = ancestor.parent
在非常高的级别,我们将输入字符串跟随到我们要删除的节点。在此之后,我们在父指针之后遍历树并删除具有1个子节点的每个节点(因为不再需要它)。一旦我们到达一个有两个孩子的节点,我们就会停止。
与Insert一样,我们可以轻松地扩充此伪代码,以便在删除失败时返回false:
如果您的Node类具有父字段,则最容易实现删除。但是,可以在没有父点的情况下实现该方法,但是更加困难。您可以看到棘手的实现here的示例。