# STROPR - Editorial

Author: Vasia Antoniuk
Tester: Istvan Nagy
Editorialist: Miguel Oliveira

Easy

Combinatorics

# PROBLEM:

Given an array A of N integers, a position x, and a number M, find the value of A_x after performing the following operation M times:

for i = 2 to N:
A[i] = A[i] + A[i-1]


As the answer could be large, the answer should be given modulo 1\,000\,000\,007.

# EXPLANATION:

In the first subtask, we have x \leq 2. So we only have 2 cases:

• x = 1: A_1 is never changed in the given procedure, so we already have the result (don’t forget the modulo!)
• x = 2: in each execution of the above procedure, we add A_1 once to A_2. So the result is just A_2 + M \times A_1.

The second subtask guarantees that N \times M \leq 10^6. This means that we can just perform the given algorithm M times. This approach takes O(N\times M) time, which is ok with these constraints.

The number N is irrelevant if larger than x because A_x is only affected by indices smaller than x. Hence, I will consider N=x for the rest of the editorial, i.e., we will want to know the value of A_N.

When we are given a set of operations to be performed, a good starting point is to see what happens after a few iterations. Suppose we have N = 5, and the initial values are A_1 = a, A_2 = b, A_3 = c, A_4 = d, and A_5 = e.

You may recognise that the factors associated with each initial value are binomial coefficients.

In each step, we are summing the values of a column in the Pascal Triangle. In the above example, M= 5, so we are interested in summing the first 5 lines in each column. For example, the value associated with a in the final A_5 is the result of \binom{3}{3} + \binom{4}{3} + \binom{5}{3} + \binom{6}{3} + \binom{7}{3}.

It can be shown (see here) that \sum_{m=0}^n{\binom{m}{k}} = \binom{n+1}{k+1}. In fact, \sum_{m=0}^7{\binom{m}{3}} = \binom{8}{4} = 70.

So, we now have a formula to get the coefficient of each initial value, in order to get the final A_N. In the above example, we have A_5 = \binom{M+3}{4} a + \binom{M+2}{3} b + \binom{M+1}{2} c + \binom{M}{1} d + \binom{M-1}{0} e = \\ \binom{8}{4} a + \binom{7}{3} b + \binom{6}{2} c + \binom{5}{1} d + \binom{4}{0} e = \\ 70a + 35b + 15c + 5d + e

The binomial coefficient can be calculated as \binom{n}{k} = \frac{n!}{k! (n-k)!}. M is up to 10^{18} so it is too big to calculate the factorial directly. However, we can take advantage of the formula we are calculating.

\binom{M+3}{4} = \frac{(M+3)!}{4! (M+3-4)!} = \frac{(M+3)!}{4! (M-1)!} = \frac{M * (M+1) * (M+2) * (M+3)}{4!}

\binom{M+2}{3} = \frac{M * (M+1) * (M+2)}{3!}

\binom{M+1}{2} = \frac{M * (M+1)}{2!}

\binom{M}{1} = \frac{M}{1!}

\binom{M-1}{0} = 1

As you can see, we can calculate each factor iteratively using the previous value. In order to divide by each factorial, we need to use the Modular Multiplicative Inverse, which can be computed in O(\log{N}). So, the time complexity of this approach is O(N \log{N}).

# Sample Solutions

4 Likes

Can make that O(n) by calculating coefficients Instantly.
https://www.codechef.com/viewsolution/9308756

Can someone help me in optimizing the code? It gives TLE for subtask 3’s test cases. https://www.codechef.com/viewsolution/9425309

@lucky30 , don’t find nCr each and every time instead use the value form the previous calculation.

1 Like

In fact, calculating the inversion from 1 to n can be O(n), … see link text

3 Likes

For an easy implementation, you may refer to My solution.

solution idea for 3 easy problems. https://shadekcse.wordpress.com/2016/02/15/codechef-feb16-challenge/

3 Likes

Nice Editorials!

1 Like

The coefficients in the solution are actually the m’th diagonal of the pascal’s triangle…

For easier and clear solution, you might want to see My solution:

https://www.codechef.com/viewsolution/9374631

2 Likes

can someone explain about the line
For example, the value associated with a in the final A5 is the result of c(3,3)+c(4,3)+c(5,3)+c(6,3)+c(7,3);?
how did you obtain this?

For a very Simple Implementation ,refer to this Solution

Can someone please point out the logical error in my code . this give WA for subtask #2 but passes all of my test cases
could be a great help for me
https://www.codechef.com/viewsolution/9455251

Can anyone tell me what’s wrong with my submission? I keep getting WA on 2 tests of the last sub task. I figured out the coefficient series as (m-1)C(m-1), (m)C(m-1), (m+1)C(m-1), (m+2)C(m-1) .... which is equivalent to the series described in the editorial. I’ve done extra mods just to be sure although that didn’t fix the WAs. Help would be greatly appreciated.