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class Solution {HashMap<Character,String> hm=new HashMap<>();List<String> res=new ArrayList<>();StringBuilder path=new StringBuilder();public List<String> letterCombinations(String digits) {if(digits==null||digits.length()==0){return res;}hm.put('1',"");hm.put('2',"abc");hm.put('3',"def");hm.put('4',"ghi");hm.put('5',"jkl");hm.put('6',"mno");hm.put('7',"pqrs");hm.put('8',"tuv");hm.put('9',"wxyz");hm.put('0',"");slout(digits,digits.length(),0);return res;}void slout(String digits,int le,int startIndex){if(path.length()==le){res.add(path.toString());return;}String st=hm.get(digits.charAt(startIndex));for(int i=0;i<st.length();i++){path.append(st.charAt(i));slout(digits,le,startIndex+1);path.deleteCharAt(path.length()-1);}}
}

class Solution(object):def letterCombinations(self, digits):hm={'1':"",'2':"abc",'3':"def",'4':"ghi",'5':"jkl",'6':"mno",'7':"pqrs",'8':"tuv",'9':"wxyz",'0':"",}res=[]path=[]def slout(digits,le,startIndex):if len(path)==le:res.append("".join(path))returnst=hm[digits[startIndex]]for i in range(len(st)):path.append(st[i])slout(digits,le,startIndex+1)path.pop()if digits is None or len(digits)==0:return resslout(digits,len(digits),0)return res