【Luogu题单】数位DP

int num[20];
ll f[20][200];
ll dfs(int pos, bool limit , ll sum){
    if(pos == 0) return sum;
    if(!limit && f[pos][sum] != -1) return f[pos][sum];
    int len = limit ? num[pos] : 9;
    ll ans = 0;
    for(int i = 0; i <= len; i ++){
        ans = (ans + dfs(pos - 1, limit && i == len, sum + i)) % mod;
    }
    if(!limit) f[pos][sum] = ans;
    return ans;
}
ll calculate(ll x){
    int pos = 0;
    while (x){
        num[++pos] = x % 10;
        x /= 10;
    }
    return dfs(pos,1 , 0);
}
void solve(){
    ll L, R;
    cin >> L >> R;
    cout << (calculate(R) - calculate(L - 1) + mod) % mod << endl;
}
int main(){
    memset(f, -1, sizeof f);
    int t;
    cin >> t;
    while (t --) solve();
    return 0;
}