509. Fibonacci Number

Tags: Easy Array

Links: https://leetcode.com/problems/fibonacci-number/

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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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class Solution {
public:
    int fib(int N) {
        if (N == 0) return 0;
        int pre = 0, cur = 1;
        for (int i = 2; i <= N; ++i) {
            int sum = cur + pre;
            pre = cur;
            cur = sum;
        }

        return cur;
    }
};

还有投机取巧的办法就是：打表：

class Solution {
public:
    int fib(int N) {
        vector<int> fibs{0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040};
        return fibs[N];
    }
};