问题描述：
In mathematics, the factorial of integer n is written as n!. It is equal to the product of n and every integer preceding it. For example: 5! = 1 x 2 x 3 x 4 x 5 = 120
Your mission is simple: write a function that takes an integer n and returns the value of n!.
You are guaranteed an integer argument. For any values outside the non-negative range, return null, nil or None (return an empty string "" in C and C++). For non-negative numbers a full length number is expected for example, return 25! = "15511210043330985984000000" as a string.
For more on factorials, see http://en.wikipedia.org/wiki/Factorial
解题思路：
刚开始就是按照寻常情况，直接就用for循环或是递归求阶乘。然后发现js的Number有位数限制（n数相对较大时，会以科学计数法呈现结果；n数很大时，越界，Infinity）。总之就不能像是题目要求的显示所有的数字。
参考博客：https://www.cnblogs.com/h5course/p/7566812.html
得出大数相乘，可以用数组来存储每一位数字，基本求解方法可以类比于小学数学乘法计算。（当24*5时，先用4*5得20，则个位数为0，进位为2；再用2*5+2得12，则十位为2，进位为1,。最后为[0,2,1]。数组倒置后即为乘法结果。）
我的答案：
function factorial(n){ ?// Add some code ?if(n<0){return null;} ?if(n==0 ||n==1){return "1";} ?let result=[1]; ??//result数组存储当前阶乘结果 ?for(let num=2;num<=n;num++){ ???for(let i=0,plus=0 ; i<result.length || plus!=0 ; i++){ ?????let count=(i<result.length)?(num*result[i]+plus):plus; //若当前i小于result所存数字的位数，则分别*num+plus;若等于，则直接进位。 ?????result[i]=count%10; ?//个位、十位、百位……上的数字，存放在数组result中 ?????plus=(count-result[i])/10; ???} ?} ?return result.reverse().join(""); //将数组result倒序后，即为最后的阶乘结果}优秀答案：
function factorial(n) { ?var res = [1]; ?for (var i = 2; i <= n; ++i) { ???var c = 0; ??//c代表进位 ???for (var j = 0; j < res.length || c !== 0; ++j) { ?????c += (res[j] || 0) * i; ?????res[j] = c % 10; //分别求出个位、十位、百位……的数 ?????c = Math.floor(c / 10); ???} ?} ?return res.reverse().join("");}另外发现直接用python做，就没有这个问题出现。(用递归方法）
def fun(n): ???if n<0: ???????return null ???elif n==0 or n==1: ???????return "1" ???else: ???????return n*fun(n-1)
用for循环
def fun(n): ???sum=1 ???if n<0: ???????return null ???elif n==0 or n==1: ???????return "1" ???else: ???????for i in range(1,n+1): ???????????sum*=i ???????return sum
哈哈哈！
codewars--js--Large Factorials--阶乘+大数阶乘
原文地址：https://www.cnblogs.com/hiluna/p/8868175.html