Sure, I can help you write a test case generator for the given problem. The generator will create random strings of varying lengths containing the characters 'A', 'B', and 'C'. Here's the code:

```python
import random

def tcgen():
    # Generate a random length for the string s
    length = random.randint(1, 200000)
    
    # Generate a random string of the given length consisting of 'A', 'B', and 'C'
    s = ''.join(random.choice('ABC') for _ in range(length))
    
    return s

# Example usage:
for _ in range(5):  # Generate 5 test cases
    print(tcgen())
```

### Explanation

1. **Length of String**: We generate a random integer between 1 and 200,000 for the length of the string, ensuring it meets the problem constraints.

2. **Generate String**: We then generate a string of the specified length composed of random characters chosen from the set {'A', 'B', 'C'}.

3. **Output**: The function returns the generated string.

This will generate test cases of varying lengths and compositions, ensuring comprehensive coverage for the problem.

### Example Output

Here's an example of what the output might look like when you run the `tcgen()` function:

```
BCABCA
A
CABCBACBACBBAC
...
```

Each string will be a random sequence of 'A', 'B', and 'C', suitable for testing the competitive programming problem's solution.