Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
'('
')'
'{'
'}'
'['
']'
The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.
"()"
"()[]{}"
"(]"
"([)]"