WA on #10 QAQ

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;

int f[5005][1005], n;

void StoBIG(string s, int a[])
{
    int len = s.size();
    for (int i = 1; i <= len; i++) a[i] = s[len - i] - '0';
    a[0] = len;
}

void printBIG(int a[])
{
    for (int i = a[0]; i >= 1; i--) cout << a[i];
    cout << endl;
}

void addBIG(int a[], int b[], int c[])
{
    int lenc = max(a[0], b[0]), u = 0;
    for (int i = 1; i <= lenc; i++)
    {
        int t = u;
        if (i <= a[0]) t += a[i];
        if (i <= b[0]) t += b[i];
        u = t / 10;
        c[i] = t % 10;
    }
    if (u > 0) c[++lenc] = u;
    c[0] = lenc;
}

int main() 
{
    cin >> n;
    StoBIG("1", f[1]);
    StoBIG("2", f[2]);
    for (int i = 3; i <= n; i++) addBIG(f[i - 1], f[i - 2], f[i]);
    printBIG(f[n]);
    return 0;
}