组合数计算的两种方法

计算(n,r)

方法一：连乘r个整商

#include<iostream>
using namespace std;
int main()
{
	int n, r;
	while (cin >> n >> r)
	{
		int ans = 1;
		for (int i = 1; i <= r; i++)
			ans =ans* n-- / i;
		cout << ans << endl;
	}
}

方法2：利用二项式计算公式（n,r)=(n-1,r)+(n-1,r-1);

#include<iostream>
#include<cstring>
using namespace std;
typedef unsigned long long int;
unsigned int c[110][110];
void pp()
{
	memset(c, 0, sizeof(c));
	c[1][1] = 1;
	for (int i = 0; i < 102; i++)
		c[i][0] = 1;
	for (int i = 1; i < 101; i++)
		for (int j = 1; j < 101; j++)
			c[i][j] = c[i - 1][j-1] + c[i - 1][j];
}
int main()
{
	int n, r;
	pp();
	while (cin >> n >> r)
		cout << c[n][r] << endl;
}