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:
"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.
Constraints:
1 <= s.length <= 2 * 104s consists of letters 'a', 'b', and 'c'