题目链接

回溯

class Solution {
    List<String> res = new ArrayList<String>();
    StringBuilder str = new StringBuilder();
    HashMap<String, String> Sites = new HashMap<String, String>();
    public List<String> letterCombinations(String digits) {
        Sites.put("0","");
        Sites.put("1","");
        Sites.put("2","abc");
        Sites.put("3","def");
        Sites.put("4","ghi");
        Sites.put("5","jkl");
        Sites.put("6","mno");
        Sites.put("7","pqrs");
        Sites.put("8","tuv");
        Sites.put("9","wxyz");
        if(digits == null || digits.length() == 0){
            return res;
        }
        backtracking(digits,0);
        return res;
    }
    public void backtracking(String digits,int index){
        if(index == digits.length()){
            res.add(str.toString());
            return;
        }
        // 将index只想的数字转为int
        int digit = digits.charAt(index) - '0';
        // 取数字对应的字符集
        String letters = Sites.get(String.valueOf(digit));
        for(int i = 0; i < letters.length(); i++){
            str.append(letters.charAt(i));
            backtracking(digits,index + 1);
            str.deleteCharAt(str.length() - 1);
        }
    }
}