Exercise: brackets

Write a function function balanced(s) that checks the brackets in the string s. It returns true if every bracket (, [ or { is later closed by a bracket of the same kind and the pairs do not cross, and false otherwise.

• balanced("{[()()]}") → true
• balanced("([)]") → false
• balanced("())") → false

Constraints: s holds from 0 to 1000 characters; the function skips every character other than the six brackets; it returns true for an empty string and false for an unclosed or extra bracket.

Do not add extra imports unless the task says so explicitly.