以下代码用于检查字符串中的括号是否匹配，横线上应填写（ ）。

def is_balanced(s):
    stack = []
    for c in s:
        if c in '([{':
            stack.append(c)
        else:
            if not stack:
                return False
            top = stack.pop()

            if (c == ')' and top != '(') or \
                (c == ']' and top != '[') or \
                (c == '}' and top != '{'):
                return False
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