# include <stdio.h>
int main( )
{
    long int a[40],i=2,n;
    for(;i<41;i++){
        a[0]=a[1]=1;
        a[i]=a[i-1]+a[i-2];
    }
    printf("请输入你需要的斐波那契数字数n(小于40)：");
    scanf("%d",&n);
    printf("该数字为%d",a[n-1]);
}
大家看看出问题在哪？

i need you help

引用a数组只能用到a[39]，i<40

哎，自己发现问题了
前边是a【40】
后边居然求了一会a【40】自己傻了

你这个算法的效率太低了  我这有个logn的算法

#include <stdio.h>
struct Matrix
{
    int _11,_12;
    int _21,_22;
};
void Ride(const Matrix *src1,const Matrix *src2,Matrix *out)
{
    out->_11 = (src1->_11*src2->_11 + src1->_12*src2->_21)%10000;
    out->_12 = (src1->_11*src2->_12 + src1->_12*src2->_22)%10000;
    out->_21 = (src1->_21*src2->_11 + src1->_22*src2->_21)%10000;
    out->_22 = (src1->_21*src2->_12 + src1->_22*src2->_22)%10000;
}
void Power(Matrix *src,int n,Matrix *out)
{
    Matrix one = {1,1,1,0};
    if(n == 1)
    {
        *out = one;
        return ;
    }
    Power(src,n>>1,out);
    Matrix temp = *out;
    Ride(&temp,&temp,out);
    if(0 == n%2)
        return ;
    else
    {
        temp = *out;
        Ride(&temp,&one,out);
    }
}
int main()
{
    int i,j,n;
    while(scanf("%d",&n) && n >=0)
    {
        if(0 == n || 1 == n)
        {
            printf("%d\n",n);
            continue;
        }
        Matrix m1 = {1,1,1,0},res;
        Power(&m1,n-1,&res);
        printf("%d\n",res._11);
    }
    return 0;
}

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