singh1495's blog

By singh1495, history, 4 years ago, In English

The Travelling Ant There is an Ant that lives in Baskerville and loves to travel. As Baskerville is a small place, it consists of only 5 cities placed one next to each other.

There is a train between each successive cities ie between City 1 — City 2, City 2 — City 3, ... City 5 — City 1. Note that our Ant loves to travel and gets happy after making exactly N train trips and returning back to home. Ant lives in the city 1 from where she begins her journey. She asks you to find the number of ways she can make N train trips and come back to home.

Since the number of ways can be huge, print that number modulo 10^9 + 7.

Input First line contains T, the number of test cases. Then T lines follow. Each line contains a single integer n, representing the number of train trips the ant needs to make.

Output For each test case, print a single line containing the answer to the problem.

Constraints 1 <= T <= 1000 0 <= n <= 10^18

Sample Input 3 0 3 4 Sample Output 1 0 6 Explanation In first case, ant has to make 0 trips. So the ant stays at city 1 and has only 1 option. In second case, ant has to make 3 trips. No matter what combination we try, we can never reach back to city 1 back after 3 trips. So answer is 0. In third case, ant makes 4 trips. There are 6 ways in which it can reach back to city 1. Way 1: 1-->2-->1-->2-->1 Way 2: 1-->2-->3-->2-->1 Way 3: 1-->5-->1-->5-->1 Way 4: 1-->5-->4-->5-->1 Way 5: 1-->5-->1-->2-->1 Way 6: 1-->2-->1-->5-->1


how can i solve this

 
 
 
 
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4 years ago, # |
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Initially, it seems like you would need to use DP to solve this question, and while the solution I'm about to present technically could be considered DP, it really appeals to a more fundamental fact about matrices.

Take your adjacency matrix of the graph of cities, which we'll call A. It should look something like:

0 1 0 0 1

1 0 1 0 0

0 1 0 1 0

0 0 1 0 1

1 0 0 1 0

Now, if we calculate An, and call this matrix B, we notice that Bij gives us the number of walks from vertex i to vertex j of length n (to see why this is true, use induction). If you use fast power, you only need to do matrix multiplications, and each multiplication will be on the order of 53 = 125, and should be well within any time limits.

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    4 years ago, # ^ |
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    thanx a-lot for giving this great solution i can find power using matrix exponentiation