------
## 最终结论
```python
def isValid(s: str) -> bool:
    t = ""
    for char in s:
        t += char
        while len(t) >= 3 and t[-3:] == "abc":
            t = t[:-3]
    return t == ""

# Example usage:
print(isValid("aabcbc"))  # Output: True
print(isValid("abcabcababcc"))  # Output: True
print(isValid("abccba"))  # Output: False
print(isValid("cababc"))  # Output: False
```