c ++ combinatorial problems

Title Description

R out problem is a combination of elements from n elements in (and regardless of order r <= n),

We can simply be understood n elements is a natural number 1,2, ..., n, which take any number r.

E.g. n = 5, r = 3, all combination:

1 2 3

1 2 4

1 2 5

1 3 4

1 3 5

1 4 5

2 3 4

2 3 5

2 4 5

3 4 5

Entry

Two row natural number n, r (1 <n <21,1 <= r <= n).

Export

All combinations, each combination per line and wherein elements are arranged in ascending order, all combinations are lexicographic order.

Sample input

5 3

Sample Output

1 2 3
1 2 4
1 2 5
1 3 4
1 3 5
1 4 5
2 3 4
2 3 5
2 4 5
3 4 5

Source Code

#include <stdio.h>
#include <iostream>
using namespace std;
bool used[30];
int ans[30];
int n,r;
void dfs(int u)
{
    if(u == r + 1)//注意:是r + 1  不是r    //如果满足R个数据,则输出
    {
        for (int i = 1;i <= r;i ++)
            printf("%d ",ans[i]);
        printf("\n");
        return ;    
    }
    for (int i = ans[u - 1] + 1;i <= n;i ++)//注意:i的起点是ans[u - 1] + 1   如果有n个数字,就循环n次来检查是否被选中
    {
        if(used[i] == 0)//如果没有被选中
        {
            ans[u] = i;//如果没有被选中,就把i放到ans[]中
            used[i] = 1;//used[2] = 1表示2这个数已经被选过了  used[3] = 1表示3这个数已经被选过了
            dfs(u + 1);//继续选下一个数字  dfs(1):选第1个数  dfs(2):选第2个数  dfs(n + 1):打印,退出
            used[i] = 0;//打印完毕,把该数字取消
        }
    }
}
int main()
{
    memset(used,0,sizeof(used));
    cin >> n;
    cin >> r;
    dfs(1);//选择第一个数字
    return 0;
}