921. Minimum Add to Make Parentheses Valid
## 思路
字串只有左右括號
所以直接預設要加n個括號, 有valid的pair就-2
## Code
```python
class Solution:
def minAddToMakeValid(self, s: str) -> int:
res = len(s)
left = 0
for ch in s:
if ch == '(':
left += 1
elif left:
left -= 1
res -= 2
return res
```