2337. Move Pieces to Obtain a String
## 思路
two pointer
每次loop 检查跳过`_`后遇到的字符
如果字符一样, 就再检查index
R: 右移 (idx_start <= idx_target
L: 左移 (idx_start >= idx_target
## Code
```python
class Solution:
def canChange(self, start: str, target: str) -> bool:
n = len(start)
i = j = 0
while True:
while i < n and start[i] == '_':
i += 1
while j < n and target[j] == '_':
j += 1
if i == n or j == n:
break
if start[i] != target[j]:
return False
if start[i] == 'L' and i < j:
return False
if start[i] == 'R' and i > j:
return False
i += 1
j += 1
return i == j == n
```