[LeetCode] 17. Letter Combinations of a Phone Number

Problem

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 <= 4
• digits[i] is a digit in the range ['2', '9'].

Solution

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> list = new LinkedList<>();
        if (digits.isEmpty()) return list;
		String[] phone = new String[]{"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
		list.add("");
		for (int i = 0; i < digits.length(); i++) {
            int num = digits.charAt(i) - 48;
			while (list.get(0).length() == i) {
				String str = list.remove(0);
				for (char ch : phone[num].toCharArray())
					list.add(str + ch);
			}
		} return list;
    }
}