Add 17_Letter_Combinations_of_a_Phone_Number.java

java/17_Letter_Combinations_of_a_Phone_Number.java

@@ -0,0 +1,40 @@
+class Solution {
+  	// make list for return.
+    public ArrayList<String> list = new ArrayList<>();
+	// make array for get phone number's characters.
+    public char[][] arr = {
+            {'0', '0', '0', '-'},
+            {'0', '0', '0', '-'},
+            {'a', 'b', 'c', '-'},
+            {'d', 'e', 'f', '-'},
+            {'g', 'h', 'i', '-'},
+            {'j', 'k', 'l', '-'},
+            {'m', 'n', 'o', '-'},
+            {'p', 'q', 'r', 's'},
+            {'t', 'u', 'v', '-'},
+            {'w', 'x', 'y', 'z'},
+        };
+
+	// main function
+    public List<String> letterCombinations(String digits) {
+        addString(digits, 0, "");
+        return list;
+    }
+
+	// axiom : if input == "", return []
+	// if index == digits.length(), add str in list
+	// else do loop number's character, and function recursion.
+    public void addString(String digits, int index, String str) {
+        if(digits.equals("")) return;
+        if(index == digits.length()) {
+            list.add(str);
+        }
+        else {
+            for(int i = 0; i < 4; i++) {
+                int number = Integer.parseInt(digits.charAt(index) + "");
+                if(arr[number][i] == '-') continue;
+                addString(digits, index + 1, str + arr[number][i]);
+            }
+        }
+    }
+}