Java Generics to Code Efficiently in Competitive Programming

Templates are the foundation of generic programming, which involve writing code in a way that is independent of any particular type. These powerful tools can be used for writing our code effectively. Some cool tricks that may be used in Competitive Programming are given as follows:

Fast Input/Output:

This uses the time advantage of BufferedReader and StringTokenizer and the advantage of user-defined methods for less typing and therefore a faster input altogether. Below is the code to find the sum of N integers using fast reader and writer.

Java

// Java program to illustrate the fast
// input output
import java.io.*;
import java.util.StringTokenizer;

public class GFG {

    // Driver Code
    public static void main(String[] args)
        throws IOException
    {

        // Initialize the reader
        FastReader reader = new FastReader();

        // Initialize the writer
        FastWriter writer = new FastWriter();

        // Your Code here

        // Reads a single integer
        int n = reader.readSingleInt();

        // Reads a array of N number
        // in a line
        int a[] = reader.readIntArray(n);

        // Prints a string
        writer.writeString("SUM OF :");

        // Print the array of number
        // with spaces
        writer.writeIntArrayWithSpaces(a);

        int sum = 0;
        for (int i = 0; i < n; i++) {
            sum += a[i];
        }

        // Prints a single number
        writer.writeSingleInteger(sum);
    }

    // Fast Reader Class
    public static class FastReader {

        // Reader object
        BufferedReader reader;

        // Constructor
        public FastReader()
        {
            // Initialize the reader
            reader = new BufferedReader(
                new InputStreamReader(
                    System.in));
        }

        // String tokenizer
        StringTokenizer tokenizer;

        // Function to read integer
        public int readSingleInt()
            throws IOException
        {
            return Integer.parseInt(
                reader.readLine());
        }

        // Function to read a single long
        public long readSingleLong()
            throws IOException
        {
            return Long.parseLong(
                reader.readLine());
        }

        // Function to read a array of
        // numsInts integers in 1 line
        public int[] readIntArray(int numInts)
            throws IOException
        {
            int[] nums = new int[numInts];
            tokenizer
                = new StringTokenizer(
                    reader.readLine());

            // Input Numbers
            for (int i = 0; i < numInts; i++) {
                nums[i] = Integer.parseInt(
                    tokenizer.nextToken());
            }
            return nums;
        }

        // Function to read string
        public String readString()
            throws IOException
        {
            return reader.readLine();
        }
    }

    // Fast Writer Class
    public static class FastWriter {

        // Writer object
        BufferedWriter writer;

        // Constructor
        public FastWriter()
        {

            // Initialize the writer
            writer = new BufferedWriter(
                new OutputStreamWriter(
                    System.out));
        }

        // Function to write single integer
        public void writeSingleInteger(int i)
            throws IOException
        {
            writer.write(Integer.toString(i));
            writer.newLine();
            writer.flush();
        }

        // Function to write a single long
        public void writeSingleLong(long i)
            throws IOException
        {
            writer.write(Long.toString(i));
            writer.newLine();
            writer.flush();
        }

        // Function to write a Integer
        // of array with spaces in 1 line
        public void writeIntArrayWithSpaces(
            int[] nums)
            throws IOException
        {
            for (int i = 0; i < nums.length; i++) {
                writer.write(nums[i] + " ");
            }
            writer.newLine();
            writer.flush();
        }

        // Function to write a Integer
        // of array without spaces
        // in 1 line
        public void writeIntArrayWithoutSpaces(int[] nums)
            throws IOException
        {
            for (int i = 0;
                 i < nums.length; i++) {
                writer.write(
                    Integer.toString(
                        nums[i]));
            }
            writer.newLine();
            writer.flush();
        }

        // Function to write a String
        public void writeString(String s)
            throws IOException
        {
            writer.write(s);
            writer.newLine();
            writer.flush();
        }
    }
}

In order to change the input and output stream based on the environment to text files or to standard input as usually done while using sublime text or other IDEs use the below code as template of FastIO.

Java

// Java program to illustrate the
// fast input output
import java.io.*;
import java.util.StringTokenizer;

public class GFG {
    public

        // Driver Code
        static void main(String[] args) throws IOException
    {

        // Initialize the reader
        FastReader reader = new FastReader();

        // Initialize the writer
        FastWriter writer = new FastWriter();

        // Your Code here
    }

    // Fast Reader Class
    public static class FastReader {

        // Reader object
        BufferedReader reader;

        // Constructor
        public FastReader()
        {

            // Initialize the reader
            reader = new BufferedReader(
                new InputStreamReader(
                    System.in));

            if (System.getProperty(
                    "ONLINE_JUDGE")
                == null) {
                try {
                    reader = new BufferedReader(
                        new InputStreamReader(
                            new FileInputStream(
                                "input.txt")));
                }
                catch (Exception e) {
                }
            }
        }

        // String tokenizer
        StringTokenizer tokenizer;

        // Function to read a
        // single integer
        public int readSingleInt()
            throws IOException
        {
            return Integer.parseInt(
                reader.readLine());
        }

        // Function to read a
        // single long
        public long readSingleLong()
            throws IOException
        {
            return Long.parseLong(
                reader.readLine());
        }

        // Function to read a array
        // of numsInts integers
        // in one line
        public int[] readIntArray(int numInts)
            throws IOException
        {
            int[] nums = new int[numInts];
            tokenizer
                = new StringTokenizer(
                    reader.readLine());

            for (int i = 0;
                 i < numInts; i++) {
                nums[i] = Integer.parseInt(
                    tokenizer.nextToken());
            }
            return nums;
        }

        // Function to read string
        public String readString()
            throws IOException
        {
            return reader.readLine();
        }
    }

    // Fast Writer Class
    public static class FastWriter {

        // Writer object
        BufferedWriter writer;

        // Constructor
        public FastWriter()
        {

            // Initialize the writer
            writer = new BufferedWriter(
                new OutputStreamWriter(
                    System.out));
            if (System.getProperty(
                    "ONLINE_JUDGE")
                == null) {
                try {
                    writer = new BufferedWriter(
                        new OutputStreamWriter(
                            new FileOutputStream(
                                "output.txt")));
                }
                catch (Exception e) {
                }
            }
        }

        // Function to write the
        // single integer
        public void writeSingleInteger(int i)
            throws IOException
        {
            writer.write(Integer.toString(i));
            writer.newLine();
            writer.flush();
        }

        // Function to write single long
        public void writeSingleLong(long i)
            throws IOException
        {
            writer.write(Long.toString(i));
            writer.newLine();
            writer.flush();
        }

        // Function to write a Integer of
        // array with spaces in one line
        public void writeIntArrayWithSpaces(int[] nums)
            throws IOException
        {
            for (int i = 0;
                 i < nums.length; i++) {
                writer.write(nums[i]
                             + " ");
            }
            writer.newLine();
            writer.flush();
        }

        // Function to write Integer of
        // array without spaces in 1 line
        public void writeIntArrayWithoutSpaces(int[] nums)
            throws IOException
        {
            for (int i = 0;
                 i < nums.length; i++) {
                writer.write(
                    Integer.toString(
                        nums[i]));
            }
            writer.newLine();
            writer.flush();
        }

        // Function to write String
        public void writeString(String s)
            throws IOException
        {
            writer.write(s);
            writer.newLine();
            writer.flush();
        }
    }
}

Note: For Information above Fast Input/Output in Java please refer to this post.

Functions commonly used in Competitive Programming:

Below are the functions that are commonly used during competitive programming, one can include them in the code to avoid wastage of time implementing it during the contest. 

Pairs in Java:

Pairs are commonly used in competitive programming. It is an easy way to use pairs in JAVA. Below is the implementation of the same:

Java

// Java program to illustrate the
// use Pairs
import java.io.*;

class GFG {

    // Driver Code
    public static void main(String[] args)
    {
        // Initialize a pair
        Pair<Integer, Integer> x
            = new Pair<Integer, Integer>(1, 2);

        // Print pair
        System.out.println(x.first + ", "
                           + x.second);
    }

    // Pair class
    static class Pair<A, B> {
        A first;
        B second;

        // Constructor
        public Pair(A first, B second)
        {
            this.first = first;
            this.second = second;
        }
    }
}

Fast Exponential using mod:

Java

// Function to find x ^ n using p as mod
static long power(long x, long y, long p)
{

    // Initialize result
    long res = 1;

    // Update x if it is more than or
    // equal to p
    x = x % p;

    while (y > 0) {

        // If y is odd, multiply x
        // with result
        if (y % 2 == 1)
            res = (res * x) % p;

        // y must be even now
        y = y >> 1; // y = y/2
        x = (x * x) % p;
    }

    return res;
}

nCr using Fermat Little Theorem:

Java

// Function to find x ^ n using p as mod
static long power(long x, long y, long p)
{

    // Initialize result
    long res = 1;

    // Update x if it is more than or
    // equal to p
    x = x % p;

    while (y > 0) {

        // If y is odd, multiply x
        // with result
        if (y % 2 == 1)
            res = (res * x) % p;

        // y must be even now
        y = y >> 1; // y = y/2
        x = (x * x) % p;
    }

    return res;
}

// Returns n^(-1) mod p
static long modInverse(long n, long p)
{
    return power(n, p - 2, p);
}

// Returns nCr % p using Fermat's
// little theorem.
static long nCrModPFermat(int n, int r, long p)
{

    // Base case
    if (r == 0)
        return 1;

    // Fill factorial array so that we
    // can find all factorial of r, n
    // and n-r
    long[] fac = new long[n + 1];
    fac[0] = 1;

    for (int i = 1; i <= n; i++)
        fac[i] = fac[i - 1] * i % p;

    return (fac[n] * modInverse(fac[r], p) % p
            * modInverse(fac[n - r], p) % p)
        % p;
}

Binomial Coefficient:

Binomial coefficient is mostly used to find the value of [n * (N – 1) *—* (N – K + 1)] / [K * (K – 1) *—-* 1]). Below is the program to implement the same:

Java

// Function to implement the
// binomial coefficient
static long binomialCoeff(long n,
                          long k,
                          long MOD)
{
    long res = 1;

    // Since C(n, k) = C(n, n-k)
    if (k > n - k)
        k = n - k;

    // Find the value of
    // [n * (n-1) *---* (n-k+1)] / [k * (k-1) *----* 1]
    for (int i = 0; i < k; ++i) {
        res *= (n - i);
        res /= (i + 1);
        res %= MOD;
    }

    // Return the result
    return res;
}

Modular Arithmetic:

Java

const int mod = 1000000007;

// Function to implement the modular
// arithmetic addition
private static long modular_add(long a, long b)
{
    return ((a % mod) + (b % mod)) % mod;
}

// Function to implement the modular
// arithmetic subtraction
private static long modular_sub(long a, long b)
{
    return ((a % mod) - (b % mod) + mod) % mod;
}

// Function to implement the modular
// arithmetic multiplication
private static long modular_mult(long a, long b)
{
    return ((a % mod) * (b % mod)) % mod;
}

Sort an array:

Java

// Function to sort an integer array
static void sort(int[] a)
{
    // Stores the element in arraylist
    ArrayList<Integer> l = new ArrayList<>();
    for (int i : a)
        l.add(i);

    // Use collection.sort() method
    Collections.sort(l);

    // Update the original array
    // with the sorted array elements
    for (int i = 0; i < a.length; i++)
        a[i] = l.get(i);
}

GCD and LCM:

Java

static long lcm(int a, int b)
{
    return (a / gcd(a, b)) * b;
}

private static long gcd(long a, long b)
{
    if (b == 0) {
        return a;
    }
    return gcd(b, a % b);
}