17. 电话号码的字母组合

[rocksc30]

class Solution {
    Stack<Character> path = new Stack<>();
    List<String> ans = new ArrayList<>();
    String[] number = new String[]{"", "", "abc", "def", "ghi",
            "jkl", "mno", "pqrs", "tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {
        if("".equals(digits)) return ans;
        helper(0, digits);
        return ans;
    }
    void helper(int n, String digits){
        if (n == digits.length()){
            ArrayList<Character> characters = new ArrayList<>(path);
            StringBuilder sb = new StringBuilder();
            characters.forEach(item -> sb.append(item));
            ans.add(sb.toString());
            return;
        }
        char c = digits.charAt(n);
        String cur = number[c - '0'];
        for (int i = 0; i < cur.length(); i++) {
            path.push(cur.charAt(i));
            helper(n + 1, digits);
            path.pop();
        }
    }
}