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.
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.
Language: 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("");
}
}
}