Sure, here's a test case generator for the given problem statement:

### Test Case Generator for the Problem Statement

```python
import random

def tcgen():
    N = random.randint(1, 70)  # Generate a random N within the specified range
    K = random.randint(1, 70)  # Generate a random K within the specified range
    s = ''.join(random.choice(['r', 'b']) for _ in range(K))  # Generate a random string of length K consisting of 'r' and 'b'
    
    return N, K, s

# Example of generating a test case
def format_test_case(N, K, s):
    return f"{N} {K}\n{s}"

# Generate a test case and print it
N, K, s = tcgen()
print(format_test_case(N, K, s))
```

### Explanation

1. **N**: Generates a random integer between 1 and 70, inclusive. This represents the number of balls.
2. **K**: Generates a random integer between 1 and 70, inclusive. This represents the length of the string `s`.
3. **s**: Creates a string of length `K` where each character is either 'r' or 'b', randomly chosen.

This ensures that the test cases generated respect the constraints given in the problem statement. 

### Example Output

Below is an example of how a test case might look when generated:

```
56 45
rrbbrrbrbbrrrbbrbrbrrbbrrrbrbbrbrrbbbrrrbbrrbb
```

This output can be directly used as input for the problem's solution function.