MJ [39] Valid Parentheses

Question:
Given a string with parentheses, return a string with balanced parentheses by removing the fewest characters possible. You cannot add anything to the string.
Examples:
balance("()") -> "()"
balance(")(") -> "".
balance("(((((") -> ""
balance("(()()(") -> "()()"
balance(")(())(") -> "(())"
Note：balance(")(())(") != "()()"

Ref
[1] http://www.mitbbs.com/article_t1/JobHunting/32913437_0_1.html