Competitive Coding Tricks

1. Multiply a number by 2

    n = n << 1

2. Divide a number by 2

    n = n >> 1

3. Check if a number is odd or even

    n & 1 ? "ODD" : "EVEN"

4. Maximum element of the array

    *max_element(a, a+n)

5. Minimum element of the array

    *min_element(a, a+n)

6. Sort elements of array in ascending order

    sort(a, a+n)

7. Sort elements of array in descending order

    sort(a, a+n, greater<int>())

8. Swap two numbers

    swap(a, b)

or you can use

    a ^= b;
    b ^= a;
    a ^= b;

9. Test Case while loop

    ll t; cin>>t; while(t--){}

10. GCD or HCF of two numbers

    __gcd(a, b)

or you can use

    ll gcd(ll a, ll b){return b ? gcd(b, a % b) : a;}

or you can use

    ll gcd(ll a,ll b){ll r; while(b){r = a % b; a = b; b = r;} return a;}

11. Number of digits in a number

    floor(log10(n)) + 1

12. Prime numbers less than n (Sieve of Eratosthenes)

    void SieveOfEratosthenes(int n){
        bool prime[n+1];
        memset(prime, true, sizeof(prime));
        for (int p = 2; p * p <= n; p++){
            if (prime[p] == true){
                for (int i = p * 2; i <= n; i += p)
                    prime[i] = false;
            }
        }
        for (int p = 2; p <= n; p++){
           if (prime[p]){
              cout << p << " ";
           }
        }
    }

13. Prime Factors of a number

    void primeFactors(int n){
        while (n % 2 == 0){
            printf("%d ", 2);
            n = n / 2;
        }
        for (int i = 3; i <= sqrt(n); i = i + 2){
            while (n % i == 0){
                cout << i;
                n = n / i;
            }
        }
        if (n > 2){
            cout << n;
        }
    }

or you can use Sieve of Eratosthenes

14. LCM of a number

    (a * b) / __gcd(a, b)

15. Largest Sum Contiguous Subarray (Kadane's Algorith)

    int maxSubArraySum(int a[], int size){
        int max_so_far = INT_MIN, max_ending_here = 0;
        for (int i = 0; i < size; i++){
            max_ending_here = max_ending_here + a[i];
            if (max_so_far < max_ending_here){
                max_so_far = max_ending_here;
            }
            if (max_ending_here < 0){
                max_ending_here = 0;
            }
        }
        return max_so_far;
    }

16. Check if a number is a power of two

    bool isPowerOfTwo(int n){
        return n && (!(n&(n-1)));
    }   

17. Copy elements of one array to another

    copy_n(source, n, destination);

18. Finding nth Root of a Number

    long double nthRoot(ll A, ll N)
    {
        // intially guessing a random number between
        // 0 and 9
        long double xPre = rand() % 10;

        //  smaller eps, denotes more accuracy
        long double eps = 1e-3;

        // initializing difference between two
        // roots by INT_MAX
        long double delX = INT_MAX;

        //  xK denotes current value of x
        long double xK;

        //  loop untill we reach desired accuracy
        while (delX > eps)
        {
            //  calculating current value from previous
            // value by newton's method
            xK = ((N - 1.0) * xPre +
                  (long double)A/pow(xPre, N-1)) / (long double)N;
            delX = abs(xK - xPre);
            xPre = xK;
        }

        return xK;
    }  

19. Getting frequency of characters in a string

    void getFrequency(string strInput,map<char,int> & fMap)
    {
         map<char,int>::iterator it; //iterator to find the entries in map
         for(int i = 0 ; i < strInput.length() ; i++ )
         {
              char ch = strInput.at(i);
              it = fMap.find(ch); //find the character in the map
              if( it == fMap.end() ) //if not present in the map
              {
                    fMap.insert( make_pair(ch,1) );//add an entry
              }
              else
              {
                    it->second++; //else increment the frequency
              }
        }
    }

    void printFrequency(map<char,int> & fMap)
    {
         map<char,int>::iterator it;//iterator to loop through all the chars
         for( it = fMap.begin() ; it != fMap.end() ; ++it )
         {
            cout<<"["<< it->first<<"]"<<"->"<<it->second<<endl;
         }
    }