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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'`

class Solution { public: bool isValid(string s) { } };