Hello Codeforces!

I often see beginners struggling with writing efficient C++ code in contests. Since time is short during competitions, knowing some handy C++ tips and tricks can save you from both Time Limit Exceeded (TLE) and bugs. Here are a few that I find most useful:

1. Fast Input / Output Instead of using cin/cout with sync, always do this: ~~~~~ ios::sync_with_stdio(false); cin.tie(nullptr); ~~~~~ This makes I/O much faster (almost as good as scanf/printf).

2. Using long long Safely

In contests, integer overflow is a common bug. Always define:

using ll = long long;

And whenever you multiply large numbers, cast them: ll result = 1LL * a * b;

3. vector vs array

->vector is flexible (dynamic size). ->array<int, N> is faster for fixed sizes.

vector<int> v(n, 0); // size n, filled with 0

**4. The Power of __gcd and std::lcm**

Instead of writing your own gcd:

#include <numeric>
int g = gcd(a, b);  // since C++17
int l = lcm(a, b);

5. Sorting and Custom Comparators

Need descending order? No need to write a custom comparator: sort(v.rbegin(), v.rend());

6. Prefix Sum and Partial Sum

Prefix sums are lifesavers:

vector<int> pref(n+1, 0);
for (int i = 0; i < n; i++) pref[i+1] = pref[i] + v[i];

7. Modular Arithmetic

When problems involve large numbers:

const int MOD = 1e9+7;
auto add = [&](int a, int b){ return (a + b) % MOD; };

Avoid overflow by using 1LL * a * b % MOD.

8. Useful Shortcuts

->memset(dp, -1, sizeof(dp)); → quick initialization. ->iota(v.begin(), v.end(), 0); → fill with 0,1,2,... ->lower_bound / upper_bound → binary search in O(log n).