leetcode17. 电话号码的字母组合

class  Solution {
    
    private List ans = new ArrayList<>();
    private List path = new ArrayList<>();
    
    
    public String getStr(char ch) {
        if (ch == '2') return "abc";
        else if (ch == '3') return "def";
        else if (ch == '4') return "ghi";
        else if (ch == '5') return "jkl";
        else if (ch == '6') return "mno";
        else if (ch == '7') return "pqrs";
        else if (ch == '8') return "tuv";
        else    return "wxyz";
    }

    public void getAns(String digits,int index) {
        if (path.size() == digits.length()) {
            StringBuilder sb = new StringBuilder(); 
            for (char ch:path) {
                sb.append(ch);
            }
            ans.add(sb.toString());

            return;
        }

        char ch = digits.charAt(index);
        String str = getStr(ch);

        for (int i = 0; i < str.length(); i++) {
            path.add(str.charAt(i));
            getAns(digits,index+1);//这里index+1表示往移动字符，也就是增加深度
            path.remove(path.size()-1);
        }

    }

    public List letterCombinations(String digits) {

        if (digits.equals("")) return ans;

       getAns(digits,0);
       return ans;
    }
}