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第 3 题:多种方式实现斐波那契数列 #9
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参考实现数学上是以递归的方法来定义 F(0) = 0;
F(1) = 1;
F(n) = F(n - 1) + F(n - 2);
function fib(n) {
if(n < 0) throw new Error('输入的数字不能小于0');
if (n < 2) {
return n;
}
return fib(n - 1) + fib(n - 2);
} 正常递归版本是一个既简单又直接的逻辑,但是这个版本有个问题就是存在大量重复计算。如:当 n 为 5 的时候要计算
function fib(n) {
if(n < 0) throw new Error('输入的数字不能小于0');
let f0 = 0,
f1 = 1,
curFib = f0;
if (n < 2) {
return n;
}
for (let i = 1; i < n; i++) {
curFib = f0 + f1;
f0 = f1;
f1 = curFib;
}
return curFib;
} 这个版本没有重复计算问题,速度也明显快了很多。这并不代表循环比递归好。循环的问题在于状态变量太多,为了实现 fib 这里使用了 4 个状态变量(f0,f1,curFib,i) 而状态变量 在写、修改、删除的过程中需要格外小心。状态变量多了阅读起来也不那么优美了。
function fib(n) {
if(n < 0) throw new Error('输入的数字不能小于0');
if (n < 2) return n;
function _fib(n, a, b) {
if (n === 0) return a;
return _fib(n - 1, b, a + b);
}
return _fib(n, 0, 1);
} 把前两位数字做成参数巧妙的避免了重复计算,性能也有明显的提升。n 做递减运算,前两位数字做递增(斐波那契数列的递增),这段代码一个减,一个增。
function* fib(n) {
if(n < 0) throw new Error('输入的数字不能小于0');
let f0 = 1,
f1 = 1,
count = 0;
while (count < n) {
yield f0;
[f0, f1] = [f1, f0 + f1];
count++;
}
}
function fib(n) {
if(n < 0) throw new Error('输入的数字不能小于0');
if (n < 2) {
return n;
}
let list = [];
list[0] = 0;
list[1] = 1;
for (let i = 1; i < n; i++) {
list[i + 1] = list[i] + list[i - 1];
}
return list[n];
} |
function fibonacci(n, map = {}) { |
export const fibonacci = (n: number): number => {
const base = Math.sqrt(5)
return (((1 + base) / 2) ** n - ((1 - base) / 2) ** n) / base
}
console.log(fibonacci(8)) |
时间复杂度 O(n) |
使用generator const createAry = function(args){
let handler = {
get: function(target,propKey,reciever){
return Reflect.get(target,propKey<0 ? (+propKey + target.length) : propKey,reciever)
},
set :function (target,propKey,value,reciever){
return Reflect.set(target,propKey<0 ? (+propKey + target.length) : propKey,value,reciever)
}
}
return new Proxy(args,handler)
}
var ary= createAry([1,2,3,4])
console.log(ary[-1]);
ary[-1]='a'
console.log(ary[-1]); |
const fibonacci = (n: number) => {
let pre = 1,
cur = 1;
if (n === 1 || n === 2) {
return 1;
}
for (let i = 3; i <= n; i++) {
[pre, cur] = [cur, pre + cur];
}
return cur;
};
console.assert(fibonacci(1) === 1);
console.assert(fibonacci(2) === 1);
console.assert(fibonacci(3) === 2);
console.assert(fibonacci(4) === 3); |
function fibonacci(n) { |
@Genzhen 写的明显有问题还那么多人点赞,笑死 |
function fabonacci(i , i1 = 1 , i2 = 1){ |
/**
/**
/**
|
function fn (n){
if(n==0) return 0
if(n==1) return 1
return fn(n-2)+fn(n-1)
}
const fn = (n)=>{
let pre = 1;
let cur = 1;
let data ;
for(var i = 3;i<=n;i++){
// 和变成当前项cur,而当前项cur会变成pre
data = pre + cur;
pre = cur
cur = data
}
return data
}
console.log(fn(2)) |
function (n) { |
|
性能最低到50就会卡死
性能大幅提升
性能进一步提升
|
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function fabonacci(n, n_1, n_2) { |
function fabonacci(max) {
if (max === 1 || max === 2) {
return 1;
}
return fabonacci(max - 1) + fabonacci(max - 2);
} |
function fibonacci(n) {
if (n === 1) {
return 1
} else if (n === 2) {
return 1
} else {
return fibonacci(n -1) + fibonacci(n - 2)
}
} |
|
function fun(length = 10, a = 0, b = 1) {
let arr = []
do {
if (arr.length < 2) {
arr.push(a)
arr.push(b)
} else {
arr.push(arr[arr.length - 2] + arr[arr.length - 1])
}
} while (arr.length < length);
return arr
}
console.log(fun(20));
// [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181] |
function fibonacci(n) {
if (n < 2) return n
let a = BigInt(0), b = BigInt(1)
for (let i = 2; i <= n; i++) {
[a, b] = [b, a + b]
}
return b
}
|
// =============无备忘录===================
function fibonacci(n) {
if (n === 1 || n === 2) { return 1 };
return fibonacci(n - 1) + fibonacci(n - 2);
}
console.log(fibonacci(30))
// =============有备忘录===================
function fibonacci_memo(n, memo = {}) {
if (n === 1 || n === 2) return 1;
if (memo[n]) return memo[n];
let data = fibonacci_memo(n - 1, memo) + fibonacci_memo(n - 2, memo);
memo[n] = data;
return data;
}
console.log(fibonacci_memo(30))
// =============for迭代===================
function fibonacci_for(n) {
if (n === 1 || n === 2) return 1;
let total = 0, cur = 1, pre = 1;
for (let i = 3; i <= n; i++) {
total = (pre + cur);
pre = cur;
cur = total;
}
return total;
}
console.log(fibonacci_for(30))
// =============generater步进===================
function fibonacci_gen(n) {
function* gen() {
let total = 0, cur = 1, pre = 1
while (1) {
total = cur + pre;
yield total;
[cur, pre] = [total, cur];
}
}
let iterator = gen();
let data = 0;
if (n === 1 || n === 2) data = 1;
else {
for (i = 3; i <= n-1; i++) {
iterator.next();
}
data = iterator.next().value
}
return data;
}
console.log(fibonacci_gen(30)) |
1 similar comment
// =============无备忘录===================
function fibonacci(n) {
if (n === 1 || n === 2) { return 1 };
return fibonacci(n - 1) + fibonacci(n - 2);
}
console.log(fibonacci(30))
// =============有备忘录===================
function fibonacci_memo(n, memo = {}) {
if (n === 1 || n === 2) return 1;
if (memo[n]) return memo[n];
let data = fibonacci_memo(n - 1, memo) + fibonacci_memo(n - 2, memo);
memo[n] = data;
return data;
}
console.log(fibonacci_memo(30))
// =============for迭代===================
function fibonacci_for(n) {
if (n === 1 || n === 2) return 1;
let total = 0, cur = 1, pre = 1;
for (let i = 3; i <= n; i++) {
total = (pre + cur);
pre = cur;
cur = total;
}
return total;
}
console.log(fibonacci_for(30))
// =============generater步进===================
function fibonacci_gen(n) {
function* gen() {
let total = 0, cur = 1, pre = 1
while (1) {
total = cur + pre;
yield total;
[cur, pre] = [total, cur];
}
}
let iterator = gen();
let data = 0;
if (n === 1 || n === 2) data = 1;
else {
for (i = 3; i <= n-1; i++) {
iterator.next();
}
data = iterator.next().value
}
return data;
}
console.log(fibonacci_gen(30)) |
|
答错题了兄弟 |
// 尾递归
function Fibonacci(n, a= 1, b= 1) {
if(n <= 1) {return b};
return Fibonacci(n - 1, b, a + b);
}
console.log(Fibonacci(10)) |
|
var fib = (n) => {
if(n <= 1) return n
return fib(n - 1) + fib(n - 2)
}
console.time('fib-1')
console.log(fib(20))
console.timeEnd('fib-1')
var fib = (n) => {
if(n <= 1) return n
fib.cache = fib.cache || {}
if(fib.cache[n]) return fib.cache[n]
fib.cache[n] = fib(n - 1) + fib(n - 2)
return fib.cache[n]
}
console.time('fib-2')
console.log(fib(20))
console.timeEnd('fib-2')
var fib = (n) => {
if(n <= 1) return n
let one = 0
let two = 1
for(let i = 2; i <= n; i++){
let o = one + two
one = two
two = o
}
return two
}
console.time('fib-3')
console.log(fib(20))
console.timeEnd('fib-3') |
// 记录fib(n - 1)和fib(n-2) 避免重复计算 从下标2开始向上计算 if (n < 2) { let f0 = 0; // 从下标为2开始 while (i <= n) { return fn; var n = 3; |
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我打印出一个数组
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这个还算稍微简单funciton fb(n) {
if(n == 1) return 1
if(n == 2) return 1
return fb(n-1) + fb(n-2)
}
funciton fb1(n) {
if(n == 1) return 1
if(n == 2) return 1
let start=1, start1 = 1,next
for(let i = 3; i<= n;i++) {
next = start + start1
start = start1
start1 = next
}
return next
} |
// 方法一:循环
function fib(n) {
let val = [0, 1];
if (n < 1) throw Error('参数n不能小于1');
if (n < 3) return val[n - 1];
for (let i = 2; i < n; i++) {
const [a, b] = val;
val = [b, a + b];
}
return val[1];
}
// 方法二:递归
function fib(n = 0, arr = [0, 1]) {
if (n < 1) throw Error('参数n不能小于1');
if (n < 3) return arr[n - 1];
const [a, b] = arr;
return fib(--n, [b, a + b]);
}
// 测试
console.log(fib(1)) // 0
console.log(fib(5)) // 3
console.log(fib(10)); // 34
// 经测试两种方法并无明显性能差别,用很大的数测试都瞬间出结果 |
let map = new Map(); console.log(fbnq(10)); |
3种方式对比 // 1.递归
// 优点:简单、易于理解
// 缺点:存在大量重复计算
function f(n) {
if (n <= 2) return 1
return f(n - 1) + f(n - 2)
}
// 2.动态规划-1
// 优点:避免了大量的重复计算
// 缺点:需要一个长度为 N 的数组空间,但其实我们只需要存储当前项的前两个值就行,所以空间可以优化到 O(1)
function f1(n) {
if (n <= 2) return 1
let dp = [1, 1]
for (let i = 2; i < n; i++) {
dp[i] = dp[i - 1] + dp[i - 2]
}
return dp[n - 1]
}
// 3.动态规划-2
// 优点:无重复计算,只需要常数个变量储存空间,空间复杂度为 O(1)
function f2(n) {
if (n <= 2) return 1
let p = 1
let q = 1
let res
for (let i = 2; i < n; i++) {
res = p + q
p = q
q = res
}
return res
} |
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欢迎在下方发表您的优质见解
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