UVa 12627 - Erratic Expansion

這題關鍵就是遞推公式，注意紫書上推的c(k)那裏貌似有點問題，應該是c(k-1)。

另外2^k方的計算可以用1<<k來快速計算，3^k可以打表。

#include <bits/stdc++.h>
using namespace std;
using LL = long long;

LL C[32] = {1};
LL g(LL k, LL i){
    if(i <= 0 || i > (1LL<<k)) return 0;
    if(k <= 0) return 1;
    if(i >= (1LL<<(k-1))) return 2 * g(k-1, i-(1LL<<(k-1))) + C[k-1];
    else return g(k-1, i);
}
int main()
{
    for(int i = 1; i < 32; ++i)
        C[i] = 3 * C[i-1];
    int T; cin >> T;
    for(int t = 1; t <= T; ++t){
        LL k, a, b; cin >> k >> a >> b;
        printf("Case %d: %lld\n", t, g(k, (1LL<<k)-a+1) - g(k, (1LL<<k)-b));
    }
    return 0;
}