## Algorithm

Problem Name: 1003. Check If Word Is Valid After Substitutions

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'`

## Code Examples

### #1 Code Example with Python Programming

```Code - Python Programming```

``````
class Solution:
def isValid(self, S: str) -> bool:
stack = []
for i in S:
if i == 'c':
if stack[-2:] != ['a', 'b']:
return False
stack.pop()
stack.pop()
else:
stack.append(i)
return not stack
``````
Copy The Code &

Input

cmd
s = "aabcbc"

Output

cmd
true