2022.04.06(LC_17_电话号码的字母组合)

 方法：回溯

class Solution {
    //数字到字母的映射
    String[] map = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    //结果集
    List<String> ans = new ArrayList<>();
    //临时路径
    StringBuilder sb = new StringBuilder();
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) return ans;
        backtracking(digits, 0);
        return ans;
    }
    public void backtracking(String digits, int index) {
        //收集结果
        if (sb.length() == digits.length()) {
            ans.add(sb.toString());
            return;
        }
        //得到数字对应的字母
        String chs = map[digits.charAt(index) - '2'];
        for (char ch : chs.toCharArray()) {
            sb.append(ch);
            //向下一个数字递归
            backtracking(digits, index + 1);
            //撤销上一次的选择
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}