letter combinations of a phone number

Author: zgpeace

code/LeetCode/src/String/LetterCombinationsOfAPhoneNumber.java

@@ -1,9 +1,6 @@
 package String;
 
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
+import java.util.*;
 
 // https://leetcode.com/problems/letter-combinations-of-a-phone-number/
 public class LetterCombinationsOfAPhoneNumber {
@@ -21,7 +18,44 @@ public class LetterCombinationsOfAPhoneNumber {
 
   public static void main(String[] args) {
     LetterCombinationsOfAPhoneNumber obj = new LetterCombinationsOfAPhoneNumber();
-    obj.letterCombinationsWithBacktrack("23");
+    obj.letterCombinationWithIterate("23");
+  }
+
+  public List<String> letterCombinationWithIterate(String digits) {
+    List<String> result = new ArrayList<>();
+    if (digits == null || digits.length() == 0) {
+      return result;
+    }
+
+    char[][] map = new char[8][];
+    map[0] = "abc".toCharArray();
+    map[1] = "def".toCharArray();
+    map[2] = "ghi".toCharArray();
+    map[3] = "jkl".toCharArray();
+    map[4] = "mno".toCharArray();
+    map[5] = "pqrs".toCharArray();
+    map[6] = "tuv".toCharArray();
+    map[7] = "wxyz".toCharArray();
+
+    result.add("");
+    for (char digit: digits.toCharArray()) {
+      result = append(result, map[digit - '2']);
+    }
+
+    System.out.println("result > " + Arrays.toString(result.toArray()));
+
+    return result;
+  }
+
+  private List<String> append(List<String> lastList, char[] charArray) {
+    List<String>  nextList = new ArrayList<>();
+    for (String s: lastList) {
+      for (char c: charArray) {
+        nextList.add(s + c);
+      }
+    }
+
+    return nextList;
   }
 
   private List<String> resultList = new ArrayList<>();

code/LeetCode/src/backtracking/LetterCombinationsOfAPhoneNumber.java

@@ -0,0 +1,111 @@
+package backtracking;
+
+import java.util.*;
+
+// https://leetcode.com/problems/letter-combinations-of-a-phone-number/
+public class LetterCombinationsOfAPhoneNumber {
+
+  Map<Character, String> phone = new HashMap<Character, String>() {{
+    put('2', "abc");
+    put('3', "def");
+    put('4', "ghi");
+    put('5', "jkl");
+    put('6', "mno");
+    put('7', "pqrs");
+    put('8', "tuv");
+    put('9', "wxyz");
+  }};
+
+  public static void main(String[] args) {
+    LetterCombinationsOfAPhoneNumber obj = new LetterCombinationsOfAPhoneNumber();
+    obj.letterCombinationWithIterate("23");
+  }
+
+  public List<String> letterCombinationWithIterate(String digits) {
+    List<String> result = new ArrayList<>();
+    if (digits == null || digits.length() == 0) {
+      return result;
+    }
+
+    char[][] map = new char[8][];
+    map[0] = "abc".toCharArray();
+    map[1] = "def".toCharArray();
+    map[2] = "ghi".toCharArray();
+    map[3] = "jkl".toCharArray();
+    map[4] = "mno".toCharArray();
+    map[5] = "pqrs".toCharArray();
+    map[6] = "tuv".toCharArray();
+    map[7] = "wxyz".toCharArray();
+
+    result.add("");
+    for (char digit: digits.toCharArray()) {
+      result = append(result, map[digit - '2']);
+    }
+
+    System.out.println("result > " + Arrays.toString(result.toArray()));
+
+    return result;
+  }
+
+  private List<String> append(List<String> lastList, char[] charArray) {
+    List<String>  nextList = new ArrayList<>();
+    for (String s: lastList) {
+      for (char c: charArray) {
+        nextList.add(s + c);
+      }
+    }
+
+    return nextList;
+  }
+
+  private List<String> resultList = new ArrayList<>();
+
+  public List<String> letterCombinationsWithBacktrack(String digits) {
+    if (digits == null || digits.length() == 0) {
+      return resultList;
+    }
+    backTrack("", digits);
+
+    return resultList;
+  }
+
+  public void backTrack(String combination, String nextDigits) {
+    if (nextDigits == null || nextDigits.length() == 0) {
+      resultList.add(combination);
+      return;
+    }
+    char c = nextDigits.charAt(0);
+    for (char item: phone.get(c).toCharArray()) {
+      backTrack(combination + item, nextDigits.substring(1));
+    }
+  }
+
+
+  public List<String> letterCombinations(String digits) {
+    // result queue
+    List<String> queue = new ArrayList<>();
+    if (digits == null || digits.length() == 0) {
+      return queue;
+    }
+    // first
+    queue.add("");
+    int gap;
+    int size;
+    String charString;
+    for (char c: digits.toCharArray()) {
+      charString = phone.get(c);
+      size = queue.size();
+      // iterate exist queue
+      for (int i = 0; i < size; i++) {
+        String s = queue.remove(0);
+        // append char
+        for (char phoneChar: charString.toCharArray()) {
+          String result = s + phoneChar;
+          queue.add(result);
+        }
+      }
+    }
+
+    return queue;
+  }
+}