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DP_CountWaysToReachNthStairUsing12Steps.java
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DP_CountWaysToReachNthStairUsing12Steps.java
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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
// https://practice.geeksforgeeks.org/problems/count-ways-to-reach-the-nth-stair/0
// https://www.geeksforgeeks.org/program-for-nth-fibonacci-number/ (Similar problem)
// https://leetcode.com/problems/climbing-stairs/ (Similar problem)
// https://www.geeksforgeeks.org/count-ways-reach-nth-stair/
// https://www.youtube.com/watch?v=CFQk7OQO_xM (BEST Video)
// https://www.youtube.com/watch?v=NFJ3m9a1oJQ
// https://www.youtube.com/watch?v=5o-kdjv7FD0
public class DP_CountWaysToReachNthStairUsing12Steps {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int t = Integer.parseInt(br.readLine().trim());
int[] fibonacciNumbers = getFibonacciNumbers(); // pre calculation of fibonacci number up to 10^5
while (t-- > 0) {
int numOfStairs = Integer.parseInt(br.readLine().trim());
/*we can pre calculate fibonacci numbers up to 10^5 and then for every query print output in O(1*/
//System.out.println(fibonacciNumbers[numOfStairs]);
System.out.println(getNumOfWaysToReachNthStair(numOfStairs + 1));
}
}
/*LOGIC : if the numOfStairs is n, then we need to find out the (n + 1)th fibonacci number*/
private static int getNumOfWaysToReachNthStair(int numOfStairs) {
int modulo = 1000000007;
int lastNumber = 1;
int secondLastNumber = 1;
int currentNumber = lastNumber + secondLastNumber;
if(numOfStairs <= 2) return lastNumber;
for (int i = 3; i <= numOfStairs; i++) {
currentNumber = (lastNumber + secondLastNumber) % modulo;
secondLastNumber = lastNumber;
lastNumber = currentNumber;
}
return currentNumber;
}
private static int[] getFibonacciNumbers() {
int modulo = 1000000007;
int[] fibonacciNumbers = new int[100001];
fibonacciNumbers[0] = 1;
fibonacciNumbers[1] = 1;
for(int i = 2; i < fibonacciNumbers.length; i++) {
fibonacciNumbers[i] = (fibonacciNumbers[i-1] + fibonacciNumbers[i-2]) % modulo;
}
return fibonacciNumbers;
}
}