Google Fresher Interview

Question:1

There is a linked list of numbers of length N. N is very large and you don’t know N. 

You have to write a function that will return k random numbers from the list. 

Numbers should be completely random.

Approach

One approach is to use random and modulo with N. But if we don't know N, it becomes interesting. The approach that we discuss is called "reservoir sampling". In this approach, we read the first K numbers and store it in the list. For our example lets say K is 10. So we will have the first 10 elements in the list. Next read the 11th element, compute a random function which returns a random number between 0 to 11. If the random number (r) is less than 10, then set the r'th number to 11th number, else continue.

Solution

Question: 2


Given a Data Structure having first n integers and next n chars. A = i1 i2 i3 ... iN c1 c2 c3 ... cN. 

Write an in-place algorithm to rearrange the elements of the array ass A = i1 c1 i2 c2 ... in cn

Approach

We can use a divide-and-conquer approach. Lets say the list contains {1, 2, 3, 4, 5, 6, 7, 8, a, b, c, d, e, f, g, h}. Using binary-search kind of approach, convert it into {1, 2, 3, 4, a, b, c, d, 5, 6, 7, 8, e, f, g, h}. Break the list into 2 parts and apply the same algorithm on the 2 parts - {1, 2, 3, 4, a, b, c, d} and {5, 6, 7, 8, e, f, g, h}. 

Solution

Question: 3

There is an array A[N] of N numbers. You have to compose an array Output[N] such that Output[i] will be equal to multiplication of all the elements of A[N] except A[i]. For example Output[0] will be multiplication of A[1] to A[N-1] and Output[1] will be multiplication of A[0] and from A[2] to A[N-1]. Solve it without division operator and in O(n).

Approach

We can do it in 2 pass, O(n). If the list is a: {4, 2, 5, 1}, we create a product list b: {0, 0, 0, 0}. Set b[0] to 1. Set pf (product forward) to 1 and pb(product backward) to 1. Start pass forward. Set pf  = pf * a[i-1]. So at i=1, pf will be 4. Set value of pf to b[i]. So the list will become {1, 4, 0, 0}. Next at i2, pf will become 4*2=8, so the list will be {1, 4, 8, 0} and next it will be {1, 4, 8, 40}.

Start pass backward from last but one item that is i=2. Set pb = pb*a[i+1]. Set value of b[i]*pb to b[i]. So pb=1. The list will be finally {10, 20, 8, 40}

Solution

Question: 4

You are given an array with integers (both positive and negative) in any random order. 

Find the sub-array with the largest sum

Solution