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.

Constraints:

- `1 <= s.length <= 2 * 104`
- `s` consists of letters `'a'`, `'b'`, and `'c'`