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: trueExplanation:"" -> "abc" -> "aabcbc" Thus, "aabcbc" is valid.

Example 2:

Input: s = "abcabcababcc"Output: trueExplanation:"" -> "abc" -> "abcabc" -> "abcabcabc" -> "abcabcababcc" Thus, "abcabcababcc" is valid.

Example 3:

Input: s = "abccba"Output: falseExplanation: It is impossible to get "abccba" using the operation.

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Constraints:

• 1 <= s.length <= 2 * 104

• s?consists of letters?'a',?'b', and?'c'

能過(guò)就行，不強(qiáng)求了。

Runtime:?635 ms, faster than?5.13%?of?Java?online submissions for?Check If Word Is Valid After Substitutions.

Memory Usage:?43 MB, less than?21.79%?of?Java?online submissions for?Check If Word Is Valid After Substitutions.