Speedcoding

Tips on how to solve an interview problem within 20 minutes.

Table of Contents

Strategy Diagram

Writing Code

Shortcuts

  • Ctrl + / comments out code; pressing again removes comments
  • Ctrl + [ increases indentation, while ] decreases indentation

Default Includes

#include <iostream>

#include <algorithm> // std::sort, std::find, std::copy, std::min_element, std::max_element
#include <functional> // std::greater
#include <span>
#include <ranges>

#include <limits>
#include <numbers>

#include <array>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <vector>
#include <tuple>
#include <utility>

#include <string>
#include <string_view>

#include <thread>
#include <mutex>
#include <memory>

#include <filesystem>

#include <any>
#include <variant>

std Namespace

Avoid typing std:: over and over again.

using namespace std;

Builtins

  • __builtin_clz returns the number of leading zeroes.

    • __builtin_ctzl for long.
    • __builtin_ctzll for long long.

  • __builtin_ctz returns the number of trailing zeroes.

    • __builtin_ctzl for long.
    • __builtin_ctzll for long long.

  • __builtin_parity returns 1 if the number is odd, 0 otherwise.

    • __builtin_parityl for long.
    • __builtin_parityll for long long.

  • __builtin_popcount returns the number of ones.

    • __builtin_popcountl for long.
    • __builtin_popcountll for long long.

auto Variable Type

auto i = 0;

auto also works in function return types, as well as lambda parameters as of C++14.

Range-based for Loop

vector<int> v = {1, 2, 3};

for (const auto& el : v) {
  cout << el;
}

Ranges

sort(v.begin(), v.end());
// vs.
ranges::sort(v);

Views

vector<int> v{1, 2, 3, 4, 5};

// You can pipe `|` multiple filters.
auto greaterthan2view = v | views::filter([](int x){return x > 2;});

Beautify Code

Use single quotes to separate digits.

iny x = 100'000'000;

Avoiding Overflow

If you're looking for a number a divisible by b then it's recommended to work in modulo b operations to avoid overflow. Please see the binary prefix divisible by 5 problem on LeetCode.

Counting Digits

floor(log10(n) + 1) // represents the number of digits in base 10 of `n`.

Testing Code

Asserts

auto a = 10;

static_assert(a == 10, "`a` should be 10 at compile-time.");

Debugging Code

Print the items from a vector:

vector<int> list;

for_each(list.begin(), list.end(), [](auto& item){ cout << item << endl; });

Print the items from an unordered map:

unordered_map<int, int> map;

for_each(map.begin(), map.end(), [](auto& item){ cout << item.first << " " << item.second << endl; });

Disabling Standard Out

Keep the cout statements in code and easily toggle them on or off.

cout.setstate(ios_base::failbit);

cout << "Hello World" << endl; // will print nothing

Improving Code

GCC Optimizations

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")
#pragma GCC optimization("max-inline-insns-recursive-auto")
#pragma GCC optimization("fast-math")

#pragma GCC target("avx,avx2,fma")

Attributes

Use [[likely]] and [[unlikely]] attributes to inform the compiler which statements are likely, and respectively unlikely to be executed more often.

if (a % 2 == 0) [[likely]] {
  cout << "`a` is even." << endl;
} else [[unlikely]] {
  cout << "`a` is odd." << endl;
}

Input Output (IO)

Makes use of lambda functions to not have C input sync with C++.

static const int _=[](){ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

Communication

ASCII Art

Using ^ to point to where the cursor is located can help interviewers understand what you're explaining.

auto s = "abcdef";
//          ^

Tricks

Moving all zeroes at the end of array.

stable_partition(nums.begin(), nums.end(), [](int n){ return n != 0; });