20. 有效的括号

class Solution:
def isValid(self, s: str) -> bool:
'''
思路：用栈实现，遇着左括号进栈，遇着与栈顶相匹配的右括号栈顶出栈
'''
dic = {
'[':']',
'(':')',
'{':'}',
}
stack = []
for i in s:
if i in ['{','(','[']:
stack.append(i) # 进栈
else:
if stack and i == dic[stack[-1]]: #当前括号与栈顶相等
stack.pop()
else:
return False
if stack: # 匹配完成后，栈中有括号就说明括号不配对
return False
else:
return True