LeetCode 509. Fibonacci Number (Easy)

Description:

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.

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Analysis:

Add memory search to speed up the program.

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Code:

class Solution {
    private Map<Integer, Integer> memo = new HashMap<>();
    public int fib(int N) {
        if(N == 0) {
            return 0;
        }else if(N == 1) {
            return 1;
        }else{
            if(memo.containsKey(N)) {
                return memo.get(N);
            }else{
                int rs = fib(N-1) + fib(N-2);
                memo.put(N, rs);
                return rs;
            }
        }
    }
}