leetcode17 电话号码的字母组合

用stringbuilder写的版本

class Solution { List<String> res = new ArrayList<>(); StringBuilder path = new StringBuilder(); String[] map = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" }; public List<String> letterCombinations(String digits) { backtrack(digits, 0); return res; } private void backtrack(String digits, int cur) { //终止条件 if (cur == digits.length()) { res.add(path.toString()); return; } //取出digits第n个数字对应的三或四个字母 String letters = map[digits.charAt(cur)-'0']; //把三个字母取出一个，然后取下一个三个字母里的一个，排列组合 for (int i = 0; i < letters.length(); i++) { path.append(letters.charAt(i)); backtrack(digits,cur+1); path.deleteCharAt(cur);//删掉末尾 } } }