LeetCode 017 Letter Combinations of a Phone Number

题目描述

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.

代码

    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);            }        }    }