> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/chapter_1_combination_and_permutation/problem_17_letter_combinations_of_a_phone_number.md).
# Problem 17: Letter Combinations of a Phone Number
>
## 思路
* 排列模板
* 先把每个数字对应的字母存到 HashMap 里，然后不停地罗列不同的排列
```java
public class Solution {
public List letterCombinations(String digits) {
List rst = new ArrayList();
if (digits == null || digits.length() == 0) {
return rst;
}
HashMap map = new HashMap();
map.put('0', new char[]{});
map.put('1', new char[]{});
map.put('2', new char[]{'a', 'b', 'c'});
map.put('3', new char[]{'d', 'e', 'f'});
map.put('4', new char[]{'g', 'h', 'i'});
map.put('5', new char[]{'j', 'k', 'l'});
map.put('6', new char[]{'m', 'n', 'o'});
map.put('7', new char[]{'p', 'q', 'r', 's'});
map.put('8', new char[]{'t', 'u', 'v'});
map.put('9', new char[]{'w', 'x', 'y', 'z'});
StringBuilder sb = new StringBuilder();
helper(digits, map, sb, rst);
return rst;
}
private void helper(String digits, HashMap map, StringBuilder sb, List rst) {
if (sb.length() == digits.length()) {
rst.add(sb.toString());
return;
}
for (char c : map.get(digits.charAt(sb.length()))) {
sb.append(c);
helper(digits, map, sb, rst);
sb.deleteCharAt(sb.length() - 1);
}
}
}
```
## 易错点
1. 初始化数组
```java
char[] arr = new char[] {'a', 'b', 'c'};
```
注意右边不要把 “\[]” 丢了
2. helper 退出条件
```java
if (sb.length() == digits.length()) {
result.add(sb.toString());
return;
}
```
3. 核心的遍历部分
```java
for (char c : map.get(digits.charAt(sb.length()))) {
sb.append(c);
helper(map, sb, digits, result);
sb.deleteCharAt(sb.length() - 1);
}
```
用`sb.length()`来 record digits 的每个字符，这个太厉害了！
`map.get()`是得到键对应的值;`digits.charAt()`得到每一个数字；然后`sb.length()`对应是第几个数字
4. StringBuilder 的方法
```java
StringBuilder sb = new StringBuilder();
sb.append('a');
sb.deleteCharAt(sb.length() - 1);
```
## Follow Up
> 在面试的时候遇到了这道题，问到了一些数组初始化的问题
1. String 数组的初始化
```java
String[] arr = new String[10];
```
String 数组初始化，每个数组内的元素都是 `null`，与 int 数组是不同的
```java
int[] arr = new int[10];
```
int 数组初始化，每个数组内的元素为 `0`
2. StringBuilder 新建后是怎么样的？
```java
StringBuilder sb = new StringBuider();
```
