Quickly multiply with 7

Give a fast method to multiply a number by 7.

Solution :

1. We multiply by 7 by first multiplying by 8 and subtracting the number from the result.
2. Multiplication by 8 is equivalent to shifting left 3 bits.

Complexity  :

• Time : O(1)
• Space : O(1)

Code  :

int multiply_7(int N){

return (N << 3) - N;

}