### What
- immutable / 不可变的
- Strings can be treated as special character [[Array|arrays]], but with dedicated operations and methods.
| Operation | Time Complexity | Description |
| ----------------- | --------------- | ---------------------------------------- |
| **Access** | O(1) | Direct access to character by index |
| **Search** | O(n) | Finding character or substring |
| **Concatenation** | O(n+m) | Joining two strings |
| **Modification** | O(n) | Due to immutability, requires rebuilding |
| **Comparison** | O(n) | Lexicographic comparison |
### ASCII
[[ASCII]]: the American Standard Code for Information Interchange
lowercase - uppercase = 32
## Grammer
### process
```python
s.replace("a", "b") # 替换
s.lower() / s.upper() # 转大小写
s.strip() / s.lstrip() / s.rstrip() # 去除空格或特定字符
s.split(",") # 拆成列表
",".join(list_of_str) # 拼成字符串
```
### transfer
```python
int("123") # 字符串 → 整数
str(456) # 整数 → 字符串
ord('a') # 得到字符的ASCII码 → 97
chr(97) # 得到ASCII码对应的字符 → 'a'
```