1. Fibonacci Number

The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

F(0) = 0,   F(1) = 1
F(N) = F(N - 1) + F(N - 2), for N > 1.

Given N, calculate F(N).

Example 1:

Input: 2
Output: 1
Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.

Example 2:

Input: 3
Output: 2
Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.

Example 3:

Input: 4
Output: 3
Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.

Note:

0 ≤ N ≤ 30.

Solution

class Solution {

public int fibo(int n, Map<Integer,Integer> m) {

if(n ==0 || n==1)
return n;

if(!m.containsKey(n)){
int sol = fibo(n-1,m) + fibo(n-2,m);
m.put(n,sol);
}

return m.get(n);

}

public int fib(int N) {
Map<Integer,Integer> m = new HashMap<>();
int sol = fibo(N,m);
return sol;
}
}