LC17. 电话号码的字母组合

 代码随想录

class Solution {

      String[] numString = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    List res = new ArrayList<>();

    public List letterCombinations(String digits) {
        if(digits == null || digits.length() == 0 ) {
            return res;
        }

        backtracking(digits,0);
        return res;


    }

    StringBuilder temp = new StringBuilder();

    public void backtracking(String digits,int num){
        if(temp.length() == digits.length()){
            res.add(temp.toString());
            return;
        }

        String str = numString[digits.charAt(num)- '0'];

        for(int i = 0 ;  i < str.length() ; i ++){
            temp.append(str.charAt(i));

            backtracking(digits,num+1);

            temp.deleteCharAt(temp.length()-1);
            
        }
    }

}