LeetCode 17 : Letter Combinations of a Phone Number

問題描述

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”].

java實習

class Solution {
    private static final String[] KEY = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<>();
        if (digits == null || digits.length() == 0)
            return result;
        backtrace(result, digits, new StringBuilder());
        return result;
    }

    private void backtrace(List<String> result, String digits, StringBuilder stringBuilder) {
        if (stringBuilder.length() == digits.length()) {
            result.add(stringBuilder.toString());
            return;
        }
        int digit = digits.charAt(stringBuilder.length()) - '0';
        String str = KEY[digit];
        for (char c : str.toCharArray()) {
            stringBuilder.append(c);
            backtrace(result, digits, stringBuilder);
            stringBuilder.deleteCharAt(stringBuilder.length() - 1);
        }
    }
}