60分求助大佬

最后两个点WA了，求助……

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

istream& operator >> (istream& in, vector<int>& a) {
    string A;
    in >> A;
    for (int i = A.size() - 1; i >= 0; --i) a.push_back(A[i] - '0');
    return in;
}

ostream& operator << (ostream& out, const vector<int>& a) {
    for (int i = a.size() - 1; i >= 0; --i) 
        out << a[i];
    return out;
}

vector<int> operator * (const vector<int>& a, const vector<int>& b) {
    vector<int> c;
    for (int i = 0; i < b.size(); i++) {
        vector<int> f(i, 0);
        int t = 0;
        for (int j = 0; j < a.size(); j++) {
            f.push_back(a[j] * b[i] + t);
            t = f[j + i] / 10;
            f[j + i] %= 10;
        }
        if (t) f.push_back(t);
        t = 0;
        vector<int> y;
        for (int j = 0; j < c.size(); j++) {
            y.push_back(c[j] + f[j] + t);
            t = y[j] / 10;
            y[j] %= 10;
        }
        for (int j = c.size(); j < f.size(); j++) 
            y.push_back(f[j] + t), t = y[j] / 10, t %= 10;
        if (t) y.push_back(t);
        c = y;
    }
    int h;
    for (h = c.size() - 1; !c[h] && h > 0; --h);
    c.resize(h + 1);
    return c; 
}

int main() {
    vector<int> a, b, c;
    cin >> a >> b;
    cout << a * b << '\n';
    return 0;
}