https://leetcode-cn.co/problems/fibonacci-number/
点击查看【bilibili】
题目
斐波那契数,通常用 F(n)
表示,形成的序列称为 斐波那契数列 。该数列由 0
和 1
开始,后面的每一项数字都是前面两项数字的和。也就是:
F(0) = 0,F(1) = 1
F(n) = F(n - 1) + F(n - 2),其中 n > 1
示例
输入:2
输出:1
解释:F(2) = F(1) + F(0) = 1 + 0 = 1
输入:3
输出:2
解释:F(3) = F(2) + F(1) = 1 + 1 = 2
输入:4
输出:3
解释:F(4) = F(3) + F(2) = 2 + 1 = 3
解答
答案
方式1,递归,自上向下,动态规划
var fib = function(n) {
if(n<=1) return n
const cache = []
cache[0] = 0
cache[1] = 1
function memoize(number){
if(cache[number] !== undefined) {
return cache[number]
}
cache[number] = memoize(number - 1) + memoize(number -2)
return cache[number]
}
const result = memoize(n)
return result
};
方式2,递归,自下向上,
var fib = function(n) {
if(n<=1) return n
const cache = []
cache[0] = 0
cache[1] = 1
for(let i=2;i<=n;i++) {
cache[i] = cache[i-1] + cache[i-2]
}
return cache[n]
};
优化空间复杂度
var fib = function(n) {
if(n<=1) return n
let prev2 = 0;
let prev1 = 1
let result = 0
for(let i=2;i<=n;i++) {
result = prev1 + prev2
prev2 = prev1
prev1 = result
}
return result
};