排列

$A_n^m$
输入：
4 2
输出：
1 2
1 3
1 4
2 1
2 3
2 4
3 1
3 2
3 4
4 1
4 2
4 3

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;int n, m;
int a[100];
bool st[100];
void dfs(int u, vector<int>& path)
{if(u == m) {for(int i = 0; i < m; i++) {cout << path[i] << " ";}cout << endl;return;}for(int i = 0; i < n; i++) {if (st[i]) continue;st[i] = true;path.push_back(a[i]);dfs(u + 1, path);path.pop_back();st[i] = false;}
}int main() {cin >> n >> m;for(int i = 0; i < n; i++) a[i] = i + 1;vector<int> path;dfs(0, path);return 0;
}

组合

$C_n^m$
输入：
5 3
输出：
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

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;int n, m;
int a[100];void dfs(int u, vector<int>& path, int start)
{if(u == m) {for(int i = 0; i < m; i++) {cout << path[i] << " ";}cout << endl;return;}for(int i = start; i < n; i++) {path.push_back(a[i]);dfs(u + 1, path, i + 1);path.pop_back();}
}int main() {cin >> n >> m;for(int i = 0; i < n; i++) a[i] = i + 1;vector<int> path;dfs(0, path, 0);return 0;
}