[USACO06FEB]数字三角形

题目链接：https://www.luogu.com.cn/problem/P1118

中文题面

想法：

当然是先暴力一发了。

然后就可以优化暴力代码了：

#include <algorithm>
#include <string>
#include <string.h>
#include <vector>
#include <map>
#include <stack>
#include <set>
#include <queue>
#include <math.h>
#include <cstdio>
#include <iomanip>
#include <time.h>
#include <bitset>
#include <cmath>
#include <sstream>
#include <iostream>
#include <cstring>

#define LL long long
#define ls nod<<1
#define rs (nod<<1)+1
#define pii pair<int,int>
#define mp make_pair
#define pb push_back

const double eps = 1e-10;
const int maxn = 5 + 10;
const LL mod = 1e9 + 7;
const LL INF = 1e18;

int sgn(double a){return a < -eps ? -1 : a < eps ? 0 : 1;}
using namespace std;

int n,sum;
int a[maxn],vis[maxn];
int c[maxn][maxn];
bool flag;

inline void dfs(int x,int s) {
    if (s > sum)
        return ;
    if (x == n+1) {
       if (s == sum) {
           flag = true;
           return ;
       }
       return ;
    }
    for (int i = 1;i <= n;i++) {
        if (!vis[i]) {
            vis[i] = 1;
            a[x] = i;
            dfs(x+1,s+i*c[n][x]);
            if (flag)
                return ;
            vis[i] = 0;
        }
    }
}


int main() {
    cin >> n >> sum;
    c[1][1] = 1;
    for (int i = 2;i <= n;i++) {
        for (int j = 1;j <= i;j++)
            c[i][j] = c[i-1][j-1]+c[i-1][j];
    }
    flag = false;
    dfs(1,0);
    if (flag) {
        for (int i = 1;i <= n;i++)
            cout << a[i] << " ";
        cout << "
";
    }
    return 0;
}