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.

Example 1:

Input: "()" Output: true 

Example 2:

Input: "()[]{}" Output: true 

Example 3:

Input: "(]" Output: false 

Example 4:

Input: "([)]" Output: false 

Example 5:

Input: "{[]}" Output: true 

括号匹配问题。

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。