[leetcode] 17. Letter Combinations of a Phone Number (medium)

递归DFS

    class Solution {
        Map<Character, String> mapping = new HashMap<>();
        public List<String> letterCombinations(String digits) {
            List<String> res = new ArrayList<>();
            if (digits.isEmpty())
                return res;
            mapping.put('2', "abc");
            mapping.put('3', "def");
            mapping.put('4', "ghi");
            mapping.put('5', "jkl");
            mapping.put('6', "mno");
            mapping.put('7', "pqrs");
            mapping.put('8', "tuv");
            mapping.put('9', "wxyz");
            char[] cdigits=digits.toCharArray();
            helper(cdigits, "", res);
            return res;
        }

        void helper(char[] cdigits, String curStr, List<String> res) {
            if (curStr.length() == cdigits.length) {
                res.add(curStr);
                return;
            }
            String curLetters = mapping.get(cdigits[curStr.length()]);
            for (char c : curLetters.toCharArray()) {
                helper(cdigits, curStr+c, res);
            }
        }
    }