-- use fread | fwrite;
 -- do not use << if >> statement or minimize it;
 -- minimize div  and mod  operations ( % , / operators);
 -- instead of % , /  operators may to use  pre-calculated arrays.
 -- and last, need lucky :)

Your program ate 12 Mb memory. So I suppose you are reading the whole numbers and then do addition.

It isn't necessary. It's possible to add numbers digit by digit (yes, from high digits to low) without any arrays usage - 0.3s even using scanf/printf.

And yes, use fread/fwrite.

Edited by author 13.12.2016 22:15

10^6 numbers --> input file size ~4 Mbytes, and output file size ~1 Mbytes.
MY PC i-core 3 , 3 GB RAM, gcc 5.4.0 with option:  g++ -Wall  -O3  -std=c++14  reads/writes took   6-9 milliseconds. So there need something magic tricky to solve this with time < 1 milliseconds !!!!!

I got 0.001s AC!!
buffered i/o  helped me.