代码重构: leetcode 17. 电话号码的字母组合

17. 电话号码的字母组合

本质上就是一个dfs，然后在过程中使用stringbuilder而不是string，减少空间的消耗

同时注意对空的处理

  private String[] strings = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<>();
        if (digits.length() == 0) return ans;
        dfs(digits, 0, new StringBuilder(), ans);
        return ans;

    }
    private void dfs(String digits, int index, StringBuilder builder, List<String> ans) {
        if (index == digits.length()) {
            ans.add(builder.toString());
            return;
        }
        String map = strings[digits.charAt(index) - '0' - 2];
        for (int i = 0; i < map.length(); i++) {
            StringBuilder sb = new StringBuilder(builder);
            sb.append(map.charAt(i));
            dfs(digits, index + 1, sb, ans);
        }
    }