Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if: Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Note that an empty string is also considered valid.

Input: "()"

Output: true

遇到左括号就进栈push，遇到右括号并且栈顶为与之对应的左括号，就把栈顶元素出栈。最后看栈⾥⾯还有没有其他元素，如果为空，即匹配。 需要注意，空字符串是满⾜括号匹配的，即输出 true。