Write a function that takes a string of brackets and checks whether they're balanced or not.

The sequence is balanced if:

• It contains no unmatched brackets.
• The subset of brackets enclosed within the confines of a matched pair of brackets is also balanced.

Examples

isBalanced("{[()]}") ➞ true

isBalanced("[()]{}") ➞ true

isBalanced("{[([)]]}") ➞ false

Notes

Return undefined if no input is given.