[USACO16JAN]Subsequences Summing to Sevens S
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. 给你n个数，分别是a,a,...,a[n]。求一个最长的区间[x,y]，使得区间中的数(a[x],a[x+1],a[x+2],...,a[y-1],a[y])的和能被7整除。输出区间长度。若没有符合要求的区间，输出0。
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.
7 3 5 1 6 2 14 10
In this example, 5+1+6+2+14 = 28.