17. Letter Combinations of a Phone Number
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
Example 1:
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
Input: digits = ""
Output: []
Example 3:
Input: digits = "2"
Output: ["a","b","c"]
Constraints:
0 <= digits.length <= 4digits[i]is a digit in the range['2', '9'].
2-9를 포함하는 문자열이 주어질 때, 해당 번호로 표현될 수 있는 모든 가능한 문자 조합을 반환합니다. 답은 어떤 순서로든 반환됩니다.
아래는 전화 버튼처럼 숫자에 대응하는 문자 매핑입니다. 1은 어떤 문자에도 대응하지 않는다는 점에 유의하세요.
예제 1:
입력: digits = "23"
출력: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
예제 2:
입력: digits = ""
출력: []
예제 3:
입력: digits = "2"
출력: ["a","b","c"]
제약사항:
0 <= digits.length <= 4digits[i]는['2', '9']범위의 숫자입니다.
Solution
import java.util.ArrayList;
import java.util.List;
public class Solution {
private static final String[] KEYPAD_MAPPINGS = {
"", // 0
"", // 1
"abc", // 2
"def", // 3
"ghi", // 4
"jkl", // 5
"mno", // 6
"pqrs", // 7
"tuv", // 8
"wxyz" // 9
};
public List<String> letterCombinations(String digits) {
List<String> results = new ArrayList<>();
if (digits == null || digits.length() == 0) {
return results;
}
backtrack(results, new StringBuilder(), digits, 0);
return results;
}
private void backtrack(List<String> combinations, StringBuilder currentCombination, String digits, int index) {
if (index == digits.length()) {
combinations.add(currentCombination.toString());
return;
}
String possibleLetters = KEYPAD_MAPPINGS[digits.charAt(index) - '0'];
for (char letter : possibleLetters.toCharArray()) {
currentCombination.append(letter);
backtrack(combinations, currentCombination, digits, index + 1);
currentCombination.deleteCharAt(currentCombination.length() - 1); // backtracking step
}
}
}
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