Google Interview Question for Web Developers


Team: Solutions Engineering Team
Country: United States
Interview Type: Phone Interview




Comment hidden because of low score. Click to expand.
2
of 2 vote

Use shiftxor algorithm to generate a random number:

#include "CommonHeader.h"

// Generate RNG numbers.
// The below initialises the seed to pre-determined number
// To get a TRNG (True-Random-Number-Generator) one should initialise the 
// seed with a true random number (e.g. capture the mouse current point and use it as the seed)

static size_t s_seed = 940983058;

void setSeed(int seed) { s_seed = seed; }

size_t rng()
{
    size_t x = s_seed;
    x ^= x << 17;
    x ^= x >> 19;
    x ^= x << 13;
    s_seed = x;
    return x;
}

size_t rng(size_t min, size_t max)
{
    if(min >= max) return std::string::npos;
    if(min == max) return min;

    size_t x = s_seed;
    x ^= x << 17;
    x ^= x >> 19;
    x ^= x << 13;
    s_seed = x;
    
    // apply the limits
    x = (x % (max - min)) + min;
    return x;
}

- PenChief September 15, 2017 | Flag Reply
Comment hidden because of low score. Click to expand.
0
of 0 vote

/**
 * @brief Generates a random integer
 * @return A random positive integer or a zero
 */
int getRandom()
{
	/* Three large prime numbers */
	/*Refer to compoasso.free.fr/primelistweb/page/prime/liste_online_en.php */
	int a = 1000003;
	int b = 2000003; // b = 2*a or closest prime number 
	int c = 3000017; // c = 3*a or closest prime number
	
	static int seed = 5;
	
	int newSeed = ((a * seed) % c + b) % c;
	if(newSeed < 0)
	{
		newSeed += c;
	}
	
	if(newSeed == seed)  // IS IT POSSIBLE ?? I DO NOT KNOW HOW TO PROVE/DISPROVE IT.
	{
		std::cout << "\nREPETITION FOUND !! " << seed << "\n";
		seed += a;
		/*Explanation for above step
		 * We are effectively doing the operation y = (ax + b) mod c, where a, b, c are primes
		 * Suppose we get some bad case when y = x [IS IT POSSIBLE ?? I DO NOT KNOW]
		 * That implies (ax + b) mod c = x
		 * Now, what if we replace x with (x + a) ? Will we still get x yet again ? NO
		 * PROOF : Let (ax + b) mod c = x
		 * To prove : It is impossible that (a(x + a) + b) mod c = x
		 * If above is possible, we can equivalently say
		 * (ax + a*a + b) mod c = x
		 * or, {(ax + b) mod c + (a*a) mod c} mod c = x
		 * or, {x  + a*a} mod c = x
		 * This is possible if and only if (a*a) is a multiple of c, which is IMPOSSIBLE because
		 * a != c, and both a and c are primes
		 * So, the logic of adding a to seed will definitely ensure that 
		 * we do not get seed again in very next iteration
		 * */
	}
	else
		seed = newSeed;
		
	return seed;
}

/**
 * @brief Generates a positive random integer in the range min to max
 * @param min Lower limit
 * @param max Upper limit
 * @return A random positive integer 
 */
int getRandom(int min, int max)
{
	if(min < 0 || max < 0 || min > max)
		throw "min must be strictly less than max";
	if(min == max)
		return min;
	else
	{
		int diff = max - min;
		return min + getRandom() % (diff + 1);
	}
}

- Interviewer September 19, 2017 | Flag Reply
Comment hidden because of low score. Click to expand.
0
of 0 vote

Check if there is anything statistically wrong with above code.
I tried it and can see approximately 51:49 distribution of 0 and 1 when running it for getRandom(0, 1).

- Interviewer September 19, 2017 | Flag Reply
Comment hidden because of low score. Click to expand.
0
of 0 vote

Check if there is anything statistically wrong with above code.
I tried it and can see approximately 51:49 distribution of 0 and 1 when running it for getRandom(0, 1).

- Interviewer September 19, 2017 | Flag Reply


Add a Comment
Name:

Writing Code? Surround your code with {{{ and }}} to preserve whitespace.

Books

is a comprehensive book on getting a job at a top tech company, while focuses on dev interviews and does this for PMs.

Learn More

Videos

CareerCup's interview videos give you a real-life look at technical interviews. In these unscripted videos, watch how other candidates handle tough questions and how the interviewer thinks about their performance.

Learn More

Resume Review

Most engineers make critical mistakes on their resumes -- we can fix your resume with our custom resume review service. And, we use fellow engineers as our resume reviewers, so you can be sure that we "get" what you're saying.

Learn More

Mock Interviews

Our Mock Interviews will be conducted "in character" just like a real interview, and can focus on whatever topics you want. All our interviewers have worked for Microsoft, Google or Amazon, you know you'll get a true-to-life experience.

Learn More