Longest prefix matching - A Trie based solution in Java | GeeksforGeeks


Longest prefix matching - A Trie based solution in Java | GeeksforGeeks
Given a dictionary of words and an input string, find the longest prefix of the string which is also a word in dictionary.

We build a Trie of all dictionary words. Once the Trie is built, traverse through it using characters of input string. If prefix matches a dictionary word, store current length and look for a longer match. Finally, return the longest match.


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;
    }
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