LeetCode 17. Letter Combinations of a Phone Number

先特判，再创建一个String数组来存储数字和字母之间的对应关系，剩下的就是经典的DFS了。

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if(digits == null || digits.length() == 0){
            return res;
        }
        String[] map = new String[] {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        backtrack(digits, 0, new StringBuilder(), map, res);
        return res;
    }
    private void backtrack(String digits, int index, StringBuilder sb, String[] map, List<String> res){
        if(index == digits.length()){
            res.add(sb.toString());
            return;
        }
        for(char c: map[digits.charAt(index) - '2'].toCharArray()){
            sb.append(c);
            backtrack(digits, index + 1, sb, map, res);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}

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