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. Example 1: Input: s = "aabcbc" Output: true Explanation: "" -> "abc" -> "aabcbc" Thus, "aabcbc" is valid. Example 2: Input: s = "abcabcababcc" Output: true Explanation: "" -> "abc" -> "abcabc" -> "abcabcabc" -> "abcabcababcc" Thus, "abcabcababcc" is valid. Example 3: Input: s = "abccba" Output: false Explanation: It is impossible to get "abccba" using the operation. Example 4: Input: s = "cababc" Output: false Explanation: It is impossible to get "cababc" using the operation. Constraints: 1 <= s.length <= 2 * 104 s consists of letters 'a', 'b', and 'c'