大数加法、乘法实现的简单版本

有时候在外需要用大数..比如面试之类，对效率要求不高，需要的是容易记住的实现，这里有一份：

#define big string
#define le length()
big addBig(big x, big y) {
    if (x.le < y.le) swap(x, y);
    // x长而y短
    for (int i = x.le - 1, j = y.le - 1; i >= 0; --i, --j) {
        if (j >= 0) x[i] += y[j] - '0';
        if (x[i] > '9') {
            x[i] -= 10;
            if (i) x[i - 1]++;
            else x = '1' + x;
        }
    }
    return x;
}
big mulInt(big s, int x) {
    reverse(s.begin(), s.end());
    int cmp = 0;
    for (int i = 0; i < (int)s.le; ++i) {
        cmp += (s[i] - '0') * x;
        s[i] = cmp % 10 + '0';
        cmp /= 10;
    }
    while (cmp) {
        s += cmp % 10 + '0';
        cmp /= 10;
    }
    reverse(s.begin(), s.end());
    return s;
}
big mulBig(big x, big y) {
    big ans;
    for (int i = y.le - 1, j = 0; i >= 0; --i, ++j) {
        big tmp = mulInt(x, y[i] - '0');
        for (int k = 0; k < j; ++k)
            tmp += '0';
        ans = addBig(ans, tmp);
    }
    return ans;
}