leetcode017 Letter Combinations of a Phone Number

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题目

17. Letter Combinations of a Phone Number
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"].

思路：

利用递归求解。

代码：

private List<String> out = new ArrayList<>();private List<String> in = new ArrayList<>();private String[] resources = {"", "", "abc", "def", "ghi", "jkl", "mon", "pqrs", "tuv", "wxyz"};public List<String> letterCombinations(String digits){    if(digits == null || digits.length() == 0 || digits.contains("1") || digits.contains("0"))        return out;    char[] c_digits = digits.toCharArray();    for(char c_digit : c_digits) in.add(resources[c_digit - '0']);    getString("", 0);    return out;}private void getString(String builder, int index){    char[] chars = in.get(index).toCharArray();    if(index == in.size() - 1)        for(char c : chars)            out.add(builder + c);    else        for(char aChar : chars)            getString(builder + aChar, index + 1);}

结果细节（图）：

0 0