isPrime, optimization 1

It is not necessary to check divisors bigger than the square root of the number.

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bool isPrime1( int x ) { int limit = (int) sqrt(x); for( int divisor = 2; divisor <= limit; divisor++ ) { if( x % divisor == 0 ) { return false; } } return true; }

• isPrime: Naive version 
• isPrime, Optimization 1: Iterate only until the square root of the number
• isPrime, Optimization 2: Only divide by odd numbers, until the square root of the number
• isPrime, Optimization 3: Skip the divisors that are multiple of 2 or 3