------
## 最终结论
```python
def longest_set_length(A):
    n = len(A)
    max_length = 0
    visited = [False] * n

    for i in range(n):
        if not visited[i]:
            current_set = set()
            current_index = i

            while current_index not in current_set:
                current_set.add(current_index)
                visited[current_index] = True
                current_index = A[current_index]

            max_length = max(max_length, len(current_set))

    return max_length

# Example usage:
A = [5, 4, 0, 3, 1, 6, 2]
print(longest_set_length(A))  # Output: 4
```