AcWing 1081. 度的数量

• 代码：

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <vector>

using namespace std;
typedef long long ll;
const ll N = 33;
const ll mod = 1000000007;
ll ans = 0;
ll K, B;
ll dp[N][N];
ll C[N][N];
void init() {
    for (int i = 0; i < N; i++) {
        for (int j = 0; j <= i; j++) {
            if (j == 0)
                C[i][j] = 1;
            else
                C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
        }
    }
}
ll DP(ll n) {
    if (n == 0) return 0;
    vector<int> nums;
    ll now = n;
    while (now) {
        nums.push_back(now % B);
        now /= B;
    }
    ll res = 0, last = 0;
    for (int i = nums.size() - 1; i >= 0; i--) {
        if (nums[i] > 0) {
            res += C[i][K - last];
            if (nums[i] > 1) {
                if (K - last - 1 >= 0) res += C[i][K - last - 1];
                break;
            } else {
                last++;
                if (last > K) break;
            }
        }
        if (!i && last == K) res++;
    }
    return res;
}
void solve() {
    ll a, b;
    cin >> a >> b >> K >> B;
    cout << DP(b) - DP(a - 1) << endl;
}
signed main() {
    ios::sync_with_stdio(0);
    ll t = 1;
    init();
    while (t--) solve();
    return 0;
}