LeetCode-in-All
17. Letter Combinations of a Phone Number
Medium
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 digit 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'].
Solution
class Solution {
List<String> letterCombinations(String digits) {
if (digits.isEmpty) {
return [];
}
final Map<String, String> hashmap = {
'2': 'abc',
'3': 'def',
'4': 'ghi',
'5': 'jkl',
'6': 'mno',
'7': 'pqrs',
'8': 'tuv',
'9': 'wxyz',
};
List<String> ans = [];
void backTracking(String digit, int index, List<String> ans) {
if (digit.length == digits.length) {
ans.add(digit);
return;
}
for (String character in hashmap[digits[index]]!.split('')) {
backTracking(digit + character, index + 1, ans);
}
}
backTracking('', 0, ans);
return ans;
}
}