这个代码竟然连输出都没有，为神马？

#include<bits/stdc++.h>
using namespace std;
const int N = 4e4 + 5;
const int M = 2e3 + 5;
int a[M], b[M], c[M];
string f[N] = {"0", "1"};
string add(string a1, string b1)
{
	memset(a, 0, sizeof(a));
	memset(b, 0, sizeof(b));
	memset(c, 0, sizeof(c));
	int lena = a1.size(), lenb = b1.size(), lenc = 1, x = 0;
	string ans = "";
	for(int i = 0; i < lena; i ++)
		a[lena - i] = a1[i] - 48;
	for(int i = 0; i < lenb; i ++)
		b[lenb - i] = b1[i] - 48;
	while(lenc <= lena || lenc <= lenb)
	{
		c[lenc] = a[lenc] + b[lenc] + x;
		x = c[lenc] / 10;
		c[lenc] %= 10;
		lenc ++;
	}
	c[lenc] = x;
	if(c[lenc] == 0)
		lenc --;
	for(int i = lenc; i >= 1; i --)
		ans += char(i + 48);
	return ans;
}
int main()
{
	//freopen(".in", "r", stdin);
	//freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int n;
	cin >> n;
	if(n <= 1)
	{
		cout << f[n];
		return 0;
	}
	for(int i = 2; i <= n; i ++)
		f[n] = add(f[n - 1], f[n - 2]);
	cout << f[n];
	return 0;
}