1003. Check If Word Is Valid After Substitutions

https://leetcode.com/problems/check-if-word-is-valid-after-substitutions

Description

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'

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