algo-aha

用杨辉三角计算组合数nCk

vector<vector<int>> C(N, vector<int>(N));
C[0][0] = 1;
for (int i = 1; i < N; i++) {
    C[i][0] = 1;
    for (int j = 1; j <= i; j++) {
        C[i][j] = C[i-1][j-1] + C[i-1][j]; // 等于上、上左的和
    }
}