why there is an extra element in cumulative sum array
?
Divisible subarray problem
this is require to apply pigeonhole principle
so that there are n holes and n+1 pigeon
after adding one extra element
we have n+1 element but distinct values that can be occured are n only form(0 ot n-1)
this is because we are taking mod with n
so we have n+1 pigeon(size of array ) and n holes( distinct values)
in this way pigeonhole principle is applied here
if you have more doubts regarding this feel free to ask
i hope this helps
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: and don’t forgot to mark doubt as resolved 