Given a string s, determine if it is valid. A string s is valid if, starting with an empty string t = " ", you can transform t into s after performing the following operation any number of times: Insert string "abc " into any position in t. More formally, t becomes tleft + "abc " + tright, where t == tleft + tright. Note that tleft and tright may be empty. Return true if s is a valid string, otherwise, return false.