A899. Subsequences Summing to Sevens--Silver

Farmer John's $N$ cows are standing in a row, as they have a tendency to do
from time to time. Each cow is labeled with a distinct integer ID number so FJ
can tell them apart. FJ would like to take a photo of a contiguous group of
cows but, due to a traumatic childhood incident involving the numbers $1
\ldots 6$, he only wants to take a picture of a group of cows if their IDs add
up to a multiple of 7.
Please help FJ determine the size of the largest group he can photograph.

The first line of input contains $N$ ($1 \leq N \leq 50,000$). The next $N$
lines each contain the $N$ integer IDs of the cows (all are in the range $0
\ldots 1,000,000$).

Please output the number of cows in the largest consecutive group whose IDs
sum to a multiple of 7. If no such group exists, output 0.
You may want to note that the sum of the IDs of a large group of cows might be
too large to fit into a standard 32-bit integer. If you are summing up large
groups of IDs, you may therefore want to use a larger integer data type, like
a 64-bit "long long" in C/C++.

In this example, 5+1+6+2+14 = 28.