Letter Combinations of a Phone N

//本题用dfs的思路解题；

class Solution { public ListletterCombinations(String digits) { //建立list 收集结果用 List resultList = new ArrayList<>();

        //判断是否为空

        if(digits == null || digits.length() == 0) return resultList;

        //建立一个具有映射关系的string数组

        String[] myMap = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

        //建立一个list 放入 映射过来的string； List digitToString = new ArrayList<>();

        for(char c : digits.toCharArray()) {

            digitToString.add(myMap[c - '0']);

        }

        int start = 0;

        StringBuilder result = new StringBuilder();

        helper(start, digitToString, resultList, result);

        return resultList;

    }

private void helper(int start, ListdigitToString, List resultList, StringBuilder result) {

        if(start == digitToString.size()) {

            resultList.add(result.toString());

            return;

        }

        for(int i = 0; i < digitToString.get(start).length(); i++) {

            result.append(digitToString.get(start).charAt(i));

            helper(start + 1, digitToString, resultList, result);

            result.deleteCharAt(result.length() - 1);

        }

    }

}