先特判，再创建一个String数组来存储数字和字母之间的对应关系，剩下的就是经典的DFS了。
class Solution {
public List<String> letterCombinations(String digits) {
List<String> res = new ArrayList<>();
if(digits == null || digits.length() == 0){
return res;
}
String[] map = new String[] {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
backtrack(digits, 0, new StringBuilder(), map, res);
return res;
}
private void backtrack(String digits, int index, StringBuilder sb, String[] map, List<String> res){
if(index == digits.length()){
res.add(sb.toString());
return;
}
for(char c: map[digits.charAt(index) - '2'].toCharArray()){
sb.append(c);
backtrack(digits, index + 1, sb, map, res);
sb.deleteCharAt(sb.length() - 1);
}
}
}
本文地址：https://blog.csdn.net/qq_31361913/article/details/107680974
