Leetcode 17.Letter Combinations of a Phone Number

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Given a digit string, 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.

Input:Digit string "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.

java

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public List<String> letterCombinations(String digits) {
    LinkedList<String> ans = new LinkedList<>();
    if (0 == digits.length()) {
        return ans;
    }
    String[] mapping = new String[]{"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    ans.add("");
    for (int i = 0; i < digits.length(); i++) {
        int x = Character.getNumericValue(digits.charAt(i));
        while (ans.peek().length() == i) {
            String temp = ans.remove();
            for (Character c : mapping[x].toCharArray()) {
                ans.add(temp + c);
            }
        }
    }
    return ans;
}

This alogirthm uses a queque to make sure every new character was added to the previous string, shown as below.