POJ 3187(Backward Digit Sums 穷竭搜索，杨辉三角性质)

//题意略
#include<cstdio>
#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<algorithm>
#include<string>
#include<cstring>
#include<cstdlib>
#include<cctype>
#include<sstream>
#include<stack>
#include<functional>
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const int INF=0x3f3f3f3f;
int sum, n;
int Cn[11];
int num[11];
bool vis[11];
bool flag;

int C(int n, int i){

return i == 0 || n == i ? 1 : C(n - 1, i - 1) + C(n - 1, i);
}

void dfs(int dep, int cursum){
if(flag || cursum > sum)return;
if(dep >= n && cursum == sum){
for(int i = 0; i < n; i++)
printf("%d%c", num[i], i == n - 1 ? '\n' : ' ');
flag = true;
}
for(int i = 1; i <= n; i++){
if(flag)return;
if(!vis[i]){
num[dep] = i;
vis[i] = true;
//杨辉三角性质
dfs(dep + 1, cursum + Cn[dep] * i);
vis[i] = false;
}
}
}
int main()
{
#ifdef LOCAL
freopen("in.txt","r",stdin);
//	freopen("out.txt","w",stdout);
#endif
//ios::sync_with_stdio(false);
cin >> n >> sum;
for(int i = 0; i < n; i++)
Cn[i] = C(n - 1, i);
flag = false;
dfs(0, 0);
return 0;
}