17. Letter Combinations of a Phone Number

Author: zhangbo54

src/leetcode/_17_/Main.java

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+package leetcode._17_;
+
+import java.util.Arrays;
+
+/**
+ * Created by zhangbo54 on 2019-03-04.
+ */
+public class Main {
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        System.out.println(solution.letterCombinations("23"));
+
+        // Integer.parseInt("1" + Integer.MAX_VALUE);
+        //  -4 -1, -1, 0, 1, 2,
+    }
+}
+

src/leetcode/_17_/Solution.java

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+package leetcode._17_;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Created by zhangbo54 on 2019-03-05.
+ */
+class Solution {
+    public List<String> letterCombinations(String digits) {
+        if (digits.length() == 0) {
+            return Collections.emptyList();
+        }
+        if (digits.length() == 1) {
+            return singleDigitToStringList(digits.charAt(0));
+        }
+        List<String> leftArr = this.letterCombinations(digits.substring(0, digits.length() / 2));
+        List<String> rightArr = this.letterCombinations(digits.substring(digits.length() / 2));
+        List<String> result = new ArrayList<>();
+        for (String left : leftArr) {
+            for (String right : rightArr) {
+                result.add(left + right);
+            }
+        }
+        return result;
+    }
+
+    private List<String> singleDigitToStringList(char digit) {
+        switch (digit) {
+            case '2':
+                return Arrays.asList("a", "b", "c");
+            case '3':
+                return Arrays.asList("d", "e", "f");
+            case '4':
+                return Arrays.asList("g", "h", "i");
+            case '5':
+                return Arrays.asList("j", "k", "l");
+            case '6':
+                return Arrays.asList("m", "n", "o");
+            case '7':
+                return Arrays.asList("p", "q", "r", "s");
+            case '8':
+                return Arrays.asList("t", "u", "v");
+            case '9':
+                return Arrays.asList("w", "x", "y", "z");
+            default:
+                return Arrays.asList("");
+        }
+    }
+}

src/leetcode/_17_/solution.md

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+### [17\. Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)
+
+Difficulty: **Medium**
+
+
+Given a string containing digits from `2-9` inclusive, 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. Note that 1 does not map to any letters.
+
+![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png)
+
+**Example:**
+
+```
+Input: "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.
+
+
+#### Solution
+
+Language: **Java**
+
+```java
+class Solution {
+    public List<String> letterCombinations(String digits) {
+        if (digits.length() == 0) {
+            return Collections.emptyList();
+        }
+        if (digits.length() == 1) {
+            return singleDigitToStringList(digits.charAt(0));
+        }
+        List<String> leftArr = this.letterCombinations(digits.substring(0, digits.length() / 2));
+        List<String> rightArr = this.letterCombinations(digits.substring(digits.length() / 2));
+        List<String> result = new ArrayList<>();
+        for (String left : leftArr) {
+            for (String right : rightArr) {
+                result.add(left + right);
+            }
+        }
+        return result;
+    }
+​
+    private List<String> singleDigitToStringList(char digit) {
+        switch (digit) {
+            case '2':
+                return Arrays.asList("a", "b", "c");
+            case '3':
+                return Arrays.asList("d", "e", "f");
+            case '4':
+                return Arrays.asList("g", "h", "i");
+            case '5':
+                return Arrays.asList("j", "k", "l");
+            case '6':
+                return Arrays.asList("m", "n", "o");
+            case '7':
+                return Arrays.asList("p", "q", "r", "s");
+            case '8':
+                return Arrays.asList("t", "u", "v");
+            case '9':
+                return Arrays.asList("w", "x", "y", "z");
+            default:
+                return Arrays.asList("");
+        }
+    }
+}
+​
+```