高精度

只需要高精加。

#include <cstdio>
#include <cstring>
#include <algorithm>
#define rep(x,y) for(int i=(x);i<=(y);i++)
struct INT{
	char d[1050];short len;
	INT(){
		std::memset(d,0,sizeof(d));
		len=1;
	};
	INT(int a){
		std::memset(d,0,sizeof(d));
		d[0]=a;
		len=1;
	}
};
INT plus_(INT a, INT b){
	INT c;
	int mxabl = std::max(a.len, b.len);
	for(int i=0;i<mxabl;i++){
		c.d[i]+=a.d[i]+b.d[i];
		if(c.d[i]>=10){
			c.d[i+1]++;
			c.d[i]-=10;
		}
	}
	if(c.d[mxabl]==0) c.len=mxabl;
	else c.len=mxabl+1;
	return c;
}

int main(){
    int n;
    std::scanf("%d",&n);
    INT a=1,b=2,c=0;
    rep(3,n){
    	c = plus_(a,b);
    	a = b;
    	b = c;
	}
    for(int i=b.len-1;i>=0;i--) std::putchar(b.d[i]+'0');
    return 0;
}