From ab414c37a3465dc074efb215429b1173ae6a5a9a Mon Sep 17 00:00:00 2001
From: yennanliu <f339339@gmail.com>
Date: Tue, 3 Oct 2023 18:48:21 +0800
Subject: [PATCH] fix 017 java

---
 .../LetterCombinationsOfAPhoneNumber.java     | 43 +++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/leetcode_java/src/main/java/LeetCodeJava/BackTrack/LetterCombinationsOfAPhoneNumber.java b/leetcode_java/src/main/java/LeetCodeJava/BackTrack/LetterCombinationsOfAPhoneNumber.java
index 203e3c5a..5e3edcf0 100644
--- a/leetcode_java/src/main/java/LeetCodeJava/BackTrack/LetterCombinationsOfAPhoneNumber.java
+++ b/leetcode_java/src/main/java/LeetCodeJava/BackTrack/LetterCombinationsOfAPhoneNumber.java
@@ -9,6 +9,49 @@
 
 public class LetterCombinationsOfAPhoneNumber {
 
+    // V0
+    List<String> _res = new ArrayList<String>();
+    public List<String> letterCombinations(String _digits) {
+
+        if (_digits.length() == 0){
+            return new ArrayList<>();
+        }
+
+        HashMap<java.lang.String, java.lang.String> letters = new HashMap<>();
+        letters.put("2", "abc");
+        letters.put("3", "def");
+        letters.put("4", "ghi");
+        letters.put("5", "jkl");
+        letters.put("6", "mno");
+        letters.put("7", "pqrs");
+        letters.put("8", "tuv");
+        letters.put("9", "wxyz");
+
+        _letter_builder(letters, 0, _digits, new StringBuilder());
+        return this._res;
+    }
+
+    private void _letter_builder(HashMap<String, String> map, int idx, String digits, StringBuilder builder){
+
+        if (builder.length() == digits.length()){
+            this._res.add(builder.toString()); // NOTE this
+            return;
+        }
+
+        String _digit = String.valueOf(digits.toCharArray()[idx]); // NOTE this
+        String _alphabets = map.get(_digit);
+
+        // backtrack
+        for (char a : _alphabets.toCharArray()){
+            builder.append(a);
+            _letter_builder(map, idx+1, digits, builder);
+            // undo
+            // builder.deleteCharAt(0); // NOTE !!! in backtrack, we remove LAST element (idx = len - 1), instead of first element
+            builder.deleteCharAt(builder.toString().length() - 1);
+        }
+    }
+
+
     // V0
     // IDEA : BACKTRACK
 //    public List<String> letterCombinations(String digits) {