memoCont

import dkh.numeric.primitive;
import dkh.modint;
alias Mint = ModInt!(10^^9+7);

struct A {
    static auto fact = factTable!Mint(100);
    static auto iFac = invFactTable!Mint(100);
    static Mint C1(int n, int k) {
        return fact[n] * iFac[k] * iFac[n-k];
    }

    // メモ化再帰でnCkの計算をする
    static memoCont!C2base C2;
    static Mint C2base(int n, int k) {
        if (k == 0) return Mint(1);
        if (n == 0) return Mint(0);
        return C2(n-1, k-1) + C2(n-1, k);
    }
}

// 0 <= n <= 99, 0 <= k <= 99, 閉区間
A.C2.init([[0, 99], [0, 99]]);
foreach (i; 0..100) {
    foreach (j; 0..i+1) {
        assert(A.C1(i, j) == A.C2(i, j));
    }
}