Most Critical Mistakes & Tips in Competitive Programming

// Java Program, to Illustrate General Tips In Competitive
// Programming

// Importing required Java libraries
import java.util.*;

public class Main {

  // Main method
  public static void main(String[] args)
  {

    // Case: Stack Overflow
    int a = 100000, b = 100000;
    // Expected answer ? 10^10 but
    // no the answer you get is 1410065408, error in
    // precision.
    int c = a * b;
    long d = (long)a * b;

    // Note: Still error will be generated because
    // calculation was done on two ints which were later
    // converted into long long ie they already
    // lost their data before converting

    // Print and display on console
    System.out.println(c + " " + d);

    // Now if we store a value more than its capacity
    // then what happens is the number line of range of
    // value becomes a number Example: Circle, If min
    // val is 1 and max is 9, so if we add 1 to 9 it
    // will result in 1, it looped back to starting,
    // this is overflow

    int p = Integer.MAX_VALUE;

    // An example of overflow
    System.out.println("An example of overflow "
                       + (p + 1));

    // Long is way better than double,
    // double although can store more than long but
    // in exchange it will cost you your precision.
    // We can simply use the below command as follows:
    // System.out.printf("%.0f", variable)
    // to give value same form as long

    // Scanner to read input
    Scanner sc = new Scanner(System.in);

    // Question where the answer came wrong coz of
    // overflow!!
    int t = sc.nextInt();
    while (t-- > 0) {
      int n = sc.nextInt();

      // Here, before i used int and got wrong answer,
      // then made it long
      long pdt = 1, temp;

      for (int i = 0; i < n; i++) {
        temp = sc.nextLong();
        pdt *= temp;
      }

      if (pdt % 10 == 2 || pdt % 10 == 3
          || pdt % 10 == 5)
        System.out.println("YES");

      else
        System.out.println("NO");
    }
  }
}

import java.util.*;

public class Main {

    public static void main(String[] args) {
        // Creating ArrayList and initializing objects
        // by passing custom integer numbers
        ArrayList<Integer> list = new ArrayList<>(
                Arrays.asList(1, 4, 2, 5, 6, 3, 9, 0, 10, 3, 15, 17, 3));

        // Operation 1
        // Finding the minimum element in the array,

        // Note: It returns the pointer/iterator
        int min = Collections.min(list);

        // Print and display elements on console
        System.out.println("Min element: " + min);

        // Operation 2
        // Finding the maximum element in the array
        int max = Collections.max(list);
        System.out.println("Max Element: " + max);

        // Operation 3
        // Sum of all elements, the third parameter tells us
        // initial sum ki value kya hai.
        int sum = 0;
        for (int val : list) {
            sum += val;
        }
        System.out.println("The sum of all elements: " + sum);

        // Operation 4
        // Count the frequency of an element in the array/list
        int cnt = Collections.frequency(list, 3);
        System.out.println("Frequency Of 3 is: " + cnt);

        // Operation 5
        // Finds an element and returns its pointer
        int index = list.indexOf(3);

        if (index != -1)
            System.out.println("the element is at posn: " + index);
        else
            System.out.println("Element not found");

        // Operation 6
        // Reversing a list using Collections.reverse method
        Collections.reverse(list);

        System.out.print("Reversed List: ");
        for (int val : list) {
            System.out.print(val + " ");
        }

        // New line for better readability
        System.out.println();

        // Operation 7
        // Sort array/list using Collections.sort() method
        Collections.sort(list);
        System.out.print("Sorted List: ");
        for (int val : list) {
            System.out.print(val + " ");
        }
    }
}