17. Letter Combinations of a Phone Number

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public static List<String> letterCombinations(String digits) {        List<String> list1 = new ArrayList<String>();        List<String> list2 = new ArrayList<String>();        List<String> list = list1;        List<String> listtmp = list2;        List<String> listcc = null;        if(digits == null || digits.length() == 0) {            return list;        }        String[] a = new String[]{                "","","abc","def",                "ghi","jkl","mno",                "pqrs","tuv","wxyz"        };        int len = digits.length();        for(int i = 0; i < len ; i++) {            String  str = a[char2num(digits.charAt(i))];            int strLen = str.length();            if(strLen != 0) {                if(!list.isEmpty()) {                    listtmp.clear();                    for(int j = 0; j < list.size(); j++) {                        String strTmp = list.get(j);                        for(int k = 0; k < strLen; k++) {                            listtmp.add(strTmp + str.charAt(k));                        }                    }                    list.clear();                    listcc = listtmp;                    listtmp = list;                    list = listcc;                }else {                    for(int j =0 ; j < strLen; j++){                        list.add(String.valueOf(a[char2num(digits.charAt(i))].charAt(j)));                    }                }            }        }        return list;    }    public static int char2num(char c) {        int i = 0;        switch(c) {            case '0' :i =0 ;break;            case '1' :i =1 ;break;            case '2' :i =2 ;break;            case '3' :i =3 ;break;            case '4' :i =4 ;break;            case '5' :i =5 ;break;            case '6' :i =6 ;break;            case '7' :i =7 ;break;            case '8' :i =8 ;break;            case '9' :i =9 ;break;        }        return i;    }
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