大数阶乘

1.将整数转换为数组

　　如123456转换成 {0,0,0,0,1,2,3,4,5,6}，数组长度根据数据规模自己决定

void convertArr(int* arr,int len,long long l)
{
    int i=0;
    int num;
    while(l!=0){
        num=l%10;
        l-=num;l/=10;
        arr[len-i-1]=num;
        i++;
    }
}

2.按乘法规则将数组元素按位相乘

void multy(int* arr1,int* arr2,int* arr3,int len)
{
    int p1=0;
    int p2=0;
    while(arr1[p1]==0) p1++;
    while(arr2[p2]==0) p2++;

    int index3=0;
    for(int i=len-1;i>=p2;i--){
        for(int j=len-1;j>=p1;j--){
            int num=arr1[j]*arr2[i]+arr3[j-index3];
            if(num<10) arr3[j-index3]=num;
            else{
                arr3[j-index3]=num%10;
                arr3[j-index3-1]+=num/10;
            }
        }
        index3++;
    }
}

3.迭代乘法

int* Factorial ( const int N,int arr1[],int len)
{

    int arr2[len];
    int arr3[len];
    for(int i=0; i<len; i++)
    {
        arr2[i]=0;
        arr3[i]=0;
    }
    convertArr(arr1,len,1);
    for(int k=2; k<=N; k++)
    {
        //arr2=k;
        convertArr(arr2,len,k);
        //arr3=arr1*arr2
        multy(arr1,arr2,arr3,len);
        //arr1=arr3
        for(int i=0; i<len; i++)
        {
            arr1[i]=arr3[i];
            arr3[i]=0;
        }
    }
    return arr1;
}

4.main函数测试

int main()
{
    int len=9000;
    int arr1[len];
    for(int i=0; i<len; i++)
    {
        arr1[i]=0;
    }
    Factorial(5,arr1,len);

    int p1=0;
    while(arr1[p1]==0) p1++;
    for(int i=p1; i<len; i++)
    {
        printf("%d",arr1[i]);
    }
    return 0;
}