17. Letter Combinations of a Phone Number

class Solution {
    List<String> result=null;
    String[] mapping = new String[]{"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public List<String> letterCombinations(String digits) {
        result = new ArrayList<>();
        if(digits.length()==0) return result;
        dfs(0,digits,new StringBuilder());
        return result;
    }
    
    void dfs(int length,String digits,StringBuilder temp){
        if(length == digits.length()){
            result.add(temp.toString());
            return;
        }
        
        //loop digit present at length index of digits string;
        char ch= digits.charAt(length);
        String str = mapping[ch-'0'];
        for(char c:str.toCharArray()){
            temp.append(c);
            dfs(length+1,digits,temp);
            temp.deleteCharAt(temp.length()-1);
        }
    }
}