Letter Combinations of a Phone Number leetcode java

題目：

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

 

題解：

這道題也是來算一種combination的題，跟 Combinations 和 Word Break II 都比較類似（其實和leetcode里面很多題都相似）。

代碼如下：

 
           1    
          public ArrayList<String> letterCombinations(String digits) { 
          
 
           2       ArrayList<String> result= 
          new ArrayList<String>(); 
          
 
           3        
          if (digits== 
          null) 
          
 
           4            
          return result; 
          
 
           5  
          
 
           6       String[] keyboard={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"}; 
          
 
           7       StringBuilder current= 
          new StringBuilder(); 
          
 
           8        
          
 
           9        
          int index=0; 
          
 
          10       buildResult(digits, index, current, keyboard, result); 
          
 
          11        
          return result; 
          
 
          12   } 
          
 
          13    
          
 
          14    
          private  
          void buildResult(String digits,  
          int index, StringBuilder current, String[] keyboard, ArrayList<String> result){ 
          
 
          15        
          if (index==digits.length()){ 
          
 
          16         result.add(current.toString()); 
          
 
          17          
          return; 
          
 
          18         } 
          
 
          19          
          
 
          20        
          int num=digits.charAt(index)-'0'; 
          // 
          get integer number 
          
 
          21  
                 
          for ( 
          int i=0; i<keyboard[num].length(); i++){ 
          
 
          22         current.append(keyboard[num].charAt(i)); 
          
 
          23         buildResult(digits, index+1, current, keyboard, result); 
          
 
          24         current.deleteCharAt(current.length()-1); 
          
 
          25         } 
          
 
          26     } 
         

 

 Refrence：http://rleetcode.blogspot.com/2014/02/letter-combinations-of-phone-number-java.html