Learning Algorithm with Yu in C++.

Bro Yu's Private Home Algorithm Cuisine.

A typical C++ code template:

#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(0);  // to speed up `cin` `cout`.
    cin.tie(0);
    /* ... */
}

Using the following command to compile the code:

g++ -std=c++11 -O2 -Wall test.cpp -o test

then, ./test to run your code.(explain: -O2 optimizes the code, -Wall shows warnings about possible errors)

Some tricks used in programming:

• the newline "\n" works faster than endl, because endl always causes a flush operation.

Input and Output

In most contests, standard streams are used for reading input and writing output.

int a, b;
string x;
cin >> a >> b >> x;

int a = 123, b = 456;
string x = "input";
cout << a << b << x << "\n";

Besides cin、cout, we can also use scanf for input, printf for output.

int a, b;
scanf("%d %d", &a, &b);

int a = 123, b = 456;
printf("%d %d\n", a, b);

For string containing spaces, we use getline:

string x;
getline(cin, x);

For the amount of data is unknown, the following loop is useful:

// loop reads elements from the input until there is no more data available.
while(cin >> x) {
    /* ... */
}

In some contest systems, files are used for input and output:

freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);

Working with numbers

Integers(long long is usually enough)

• int: -2^31...2^31 - 1 or about -2*10^9...2*10^9
• long long: -2^63...2^63 - 1 or about -9*10^18...9*10^18
• __int128_t: -2^127...2^127 - 1 or about -10^38...10^38(may not available for all contest systems)

Modular arithmetic

(a + b) mod m = (a mod m + b mod m) mod m
(a − b) mod m = (a mod m − b mod m) mod m
(a · b) mod m = (a mod m · b mod m) mod m

make sure there are no negative remainders:

if (x < 0) x += m;

Floating point numbers
The usual floating point types in competitive programming are the 64-bit double and, as an extension in the g++ compiler, the 80-bit long double. In most cases, double is enough, but long double is more accurate.

An easy way to output the answer that required precision is to use the printf function and give the number of decimal places in the formatting string.

printf("%.9f\n", x);

Compare two float point number(to eliminate the round error):

if(abs(a - b) < 1e-9) {
    /* a and b are equal */
}

Shortening code

Type names

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;

Macros

#define F first
#define S second
#define PB push_back
#define MP make_pair

#define REP(i, a, b) for(int i = a; i < b; ++i)
#define SQ(a) (a)*(a)

// now
for(int i = 1; i < n; ++i) {
    search(i);
}
// become 
REP(i, 1, n) {
    search(i);
}

Recommended books for coding competition & tech-interview

1. Competitive Programmer’s Handbook(Free PDF) ⭐
2. Cracking the Coding Interview 6th Edition: 189 Programming Questions and Solutions