C recursion method as Fibonacci

struct mapStruct
{
    int key;
    unsigned long long value;
};

static struct mapStruct arr[1000];

unsigned long long fab(unsigned long long x);
void fab13();

int main()
{
   fab13();
}

void fab13()
{
    unsigned long long result=fab(100);
    printf("x=%d,result=%llu\n",10,result);
}

unsigned long long fab(unsigned long long x)
{    
    unsigned long long result;
    result=arr[x].value;
    if(result>0)
    {
        return result;
    }
    if(x<=2)
    {
        result=1;
    }
    else
    {
        result= fab(x-1)+fab(x-2);
    }   
    arr[x].key=x;
    arr[x].value=result; 
    return result;
}

This has the optimization for original recurison,declare a struct and struct array to store the temp calculated result which will avoid redundant computions.

struct mapStruct
{
int key;
unsigned long long value;
};

static struct mapStruct arr[1000];

 

At first it will ask the value from the static arr to detect whether its Fibonacci's value has stored in the struct array.If true,it will return the result directly and no need to run next steps;

When it can not retrieve value or the Fibonacci's value it will run the conditional way!