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  1. import java.util.ArrayList;
  2. import java.util.HashMap;
  3. import java.util.Scanner;
  4.  
  5. class Source {
  6.  
  7.     public ArrayList<String> letterCombinations(String digits) {
  8.         ArrayList<String> result = new ArrayList<>();
  9.  
  10.         // Edge case: empty input
  11.         if (digits == null || digits.length() == 0) {
  12.             return result;
  13.         }
  14.  
  15.         // Digit-to-letter mapping
  16.         HashMap<Character, String> map = new HashMap<>();
  17.         map.put('2', "abc");
  18.         map.put('3', "def");
  19.         map.put('4', "ghi");
  20.         map.put('5', "jkl");
  21.         map.put('6', "mno");
  22.         map.put('7', "pqrs");
  23.         map.put('8', "tuv");
  24.         map.put('9', "wxyz");
  25.  
  26.         // Check for invalid digits like 0,1 or non-numeric
  27.         for (char ch : digits.toCharArray()) {
  28.             if (!map.containsKey(ch)) {
  29.                 return result; // return [] if invalid input
  30.             }
  31.         }
  32.  
  33.         backtrack(result, map, digits, 0, new StringBuilder());
  34.         return result;
  35.     }
  36.  
  37.     private void backtrack(ArrayList<String> result, HashMap<Character, String> map,
  38.                            String digits, int index, StringBuilder current) {
  39.         if (index == digits.length()) {
  40.             result.add(current.toString());
  41.             return;
  42.         }
  43.  
  44.         char digit = digits.charAt(index);
  45.         String possibleLetters = map.get(digit);
  46.  
  47.         for (char letter : possibleLetters.toCharArray()) {
  48.             current.append(letter);
  49.             backtrack(result, map, digits, index + 1, current);
  50.             current.deleteCharAt(current.length() - 1);
  51.         }
  52.     }
  53.  
  54.     // Only for local/SPOJ testing
  55.     public static void main(String[] args) {
  56.         Scanner sc = new Scanner(System.in);
  57.         String digits = sc.nextLine().trim();
  58.  
  59.         Source src = new Source();
  60.         ArrayList<String> ans = src.letterCombinations(digits);
  61.  
  62.         System.out.println(ans);
  63.     }
  64. }
  65.  
Success #stdin #stdout 0.18s 54628KB
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stdout
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[ad, ae, af, bd, be, bf, cd, ce, cf]
https://ideone.com/RjBCkf
.../lua-eigen/src/matrix.hpp:338: multiple definition of `LuaEigen::Matrix<float, -1, -1>::init(lua_State*)'
CMakeFiles/eigen.dir/src/eigen.cpp.o:.../lua-eigen/src/matrix.hpp:338: first defined here
language:
Java (12.0.1)
created:
9 hours ago
visibility:
public

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