[LeetCode] 921. Minimum Add to Make Parentheses Valid 使括号有效的最少添加
GivenastringSof'('and')'parentheses,weaddtheminimumnumberofparentheses('('or')',andinanypositions)sothattheresultingparenthesesstringisvalid.Formally,aparenthesesstringisvalidifandonlyif:Itistheemptys