给定一个单词字典和一个输入字符串，找出该字符串的最长前缀，该前缀也是字典中的一个单词。
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例如：
Let the dictionary contains the following words:
{are, area, base, cat, cater, children, basement}
Below are some input/output examples:
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Input String Output
--------------------------------------
caterer cater
basemexy base
child < Empty >
解决方案 我们建立了一个包含所有字典单词的Trie。构建Trie后，使用输入字符串的字符遍历它。如果前缀与词典中的单词匹配，请存储当前长度并查找更长的匹配项。最后，返回最长的比赛。 以下是基于上述解决方案的Java实现。
import java.util.HashMap; // Trie Node, which stores a character and the children in a HashMap class TrieNode { public TrieNode( char ch) { value = ch; children = new HashMap<>(); bIsEnd = false ; } public HashMap<Character,TrieNode> getChildren() { return children; } public char getValue() { return value; } public void setIsEnd( boolean val) { bIsEnd = val; } public boolean isEnd() { return bIsEnd; } private char value; private HashMap<Character,TrieNode> children; private boolean bIsEnd; } // Implements the actual Trie class Trie { // Constructor public Trie() { root = new TrieNode(( char ) 0 ); } // Method to insert a new word to Trie public void insert(String word) { // Find length of the given word int length = word.length(); TrieNode crawl = root; // Traverse through all characters of given word for ( int level = 0 ; level < length; level++) { HashMap<Character,TrieNode> child = crawl.getChildren(); char ch = word.charAt(level); // If there is already a child for current character of given word if ( child.containsKey(ch)) crawl = child.get(ch); else // Else create a child { TrieNode temp = new TrieNode(ch); child.put( ch, temp ); crawl = temp; } } // Set bIsEnd true for last character crawl.setIsEnd( true ); } // The main method that finds out the longest string 'input' public String getMatchingPrefix(String input) { String result = "" ; // Initialize resultant string int length = input.length(); // Find length of the input string // Initialize reference to traverse through Trie TrieNode crawl = root; // Iterate through all characters of input string 'str' and traverse // down the Trie int level, prevMatch = 0 ; for ( level = 0 ; level < length; level++ ) { // Find current character of str char ch = input.charAt(level); // HashMap of current Trie node to traverse down HashMap<Character,TrieNode> child = crawl.getChildren(); // See if there is a Trie edge for the current character if ( child.containsKey(ch) ) { result += ch; //Update result crawl = child.get(ch); //Update crawl to move down in Trie // If this is end of a word, then update prevMatch if ( crawl.isEnd() ) prevMatch = level + 1 ; } else break ; } // If the last processed character did not match end of a word, // return the previously matching prefix if ( !crawl.isEnd() ) return result.substring( 0 , prevMatch); else return result; } private TrieNode root; } // Testing class public class Test { public static void main(String[] args) { Trie dict = new Trie(); dict.insert( "are" ); dict.insert( "area" ); dict.insert( "base" ); dict.insert( "cat" ); dict.insert( "cater" ); dict.insert( "basement" ); String input = "caterer" ; System.out.print(input + ": " ); System.out.println(dict.getMatchingPrefix(input)); input = "basement" ; System.out.print(input + ": " ); System.out.println(dict.getMatchingPrefix(input)); input = "are" ; System.out.print(input + ": " ); System.out.println(dict.getMatchingPrefix(input)); input = "arex" ; System.out.print(input + ": " ); System.out.println(dict.getMatchingPrefix(input)); input = "basemexz" ; System.out.print(input + ": " ); System.out.println(dict.getMatchingPrefix(input)); input = "xyz" ; System.out.print(input + ": " ); System.out.println(dict.getMatchingPrefix(input)); } } |
输出：
caterer: cater basement: basement are: are arex: are basemexz: base xyz:
时间复杂度：查找最长前缀的时间复杂度为O（n），其中n是输入字符串的长度。参考 这 因为建造Trie的时间复杂性。
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