Fast IO and the Contest Template

Making cin fast enough for big inputs

Judge problems can feed you hundreds of thousands of numbers. Two lines at the top of main make C++ streams read them fast:

std::ios::sync_with_stdio(false);  // stop syncing with C's stdio
std::cin.tie(nullptr);             // stop flushing cout before every cin

And prefer "\n" over std::endl (lesson 1-2): endl forces a flush on every line, which is slow in a tight loop.

Two more habits

Read until the input ends. std::cin >> x evaluates to false when input runs out, so an unknown-length stream is just:

int x;
while (std::cin >> x) { /* use x */ }

You used this in lesson 9-1's word counter.

Sum in long long. An int caps near 2.1 billion (lesson 2-1). Summing 100,000 values that can each be a billion overflows int silently. long long holds about ±9.2 × 10¹⁸. In contests, when in doubt, sums and products are long long.