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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

思路：典型backtracking，建立一个mapping的数组，然后进行NP搜索，注意最后递归结束条件是sb.length() == digits.length()

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> list = new ArrayList<>();
        if(digits == null || digits.length() == 0) {
            return list;
        }
        String[] patterns = new String[] {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        StringBuilder sb = new StringBuilder();
        dfs(patterns, digits, 0, sb, list);
        return list;
    }
    
    private void dfs(String[] patterns, String digits, int index, StringBuilder sb, List<String> list) {
        if(index == digits.length()) {
            list.add(sb.toString());
            return;
        }
        int digit = digits.charAt(index) - '0';
        String pattern = patterns[digit];
        for(int i = 0; i < pattern.length(); i++) {
            sb.append(pattern.charAt(i));
            dfs(patterns, digits, index + 1, sb, list);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}