Leetcode 题目解析之 Letter Combinations of a Phone Number

    static final char[][] CHAR_MAP = { {},// 0
            {},// 1
            { 'a', 'b', 'c' },// 2
            { 'd', 'e', 'f' },// 3
            { 'g', 'h', 'i' },// 4
            { 'j', 'k', 'l' },// 5
            { 'm', 'n', 'o' },// 6
            { 'p', 'q', 'r', 's' },// 7
            { 't', 'u', 'v' },// 8
            { 'w', 'x', 'y', 'z' } // 9
    };
    List<String> result;
    char[] stack;
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) {
            return new ArrayList<String>();
        }
        result = new ArrayList<String>();
        stack = new char[digits.length()];
        dfs(digits.toCharArray(), 0);
        return result;
    }
    private void dfs(char[] digits, int p) {
        if (p == digits.length) {
            result.add(new String(stack));
        } else {
            int num = digits[p] - '0';
            for (char c : CHAR_MAP[num]) {
                stack[p] = c;
                dfs(digits, p + 1);
            }
        }
    }