https://ac.nowcoder.com/acm/contest/11254/F

一道标准的搜索题。
注意题目有坑点，其他按照搜索即可。
我的写法中包含两层搜索+精度的控制。

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <stack>
#include <algorithm>
#include <map>
#include <stdio.h>

using namespace std;

struct node
{
    vector<int>line;
};
vector<node>ans;
stack<int>tmp;
double mm;


int aaaa =0 ;
void dfsans(vector<double> &now,int chu)
{
    if(now.size()==1&&abs(now[0]-mm)<0.00001&&chu>0) {aaaa++;return ;}
    if(now.size()==1&&abs(now[0]-mm)<0.00001) {aaaa=-1000000;return ;}
    if(now.size()==1) return ;

    vector<double> nex;
    for (size_t i = 0; i < now.size()-1; i++)
    {
        nex.push_back(now[i]);
    }

    int t;
    for (size_t i = 0; i < now.size() && aaaa >=0 ; i++)
    {
        for (size_t j = 0; j < now.size() && aaaa >=0; j++)
        {

            if (i==j)
            {
                continue;
            }
            
            nex[0] = now[i] + now[j];t=1;
            for (size_t k = 0; k < now.size(); k++)
            {
                if (k!=i && k!=j)
                {
                    nex[t] =now[k];
                    t ++;
                }
                
            }
            dfsans(nex,chu);

            nex[0] = now[i] - now[j];
            dfsans(nex,chu);

            nex[0] = now[i] * now[j];
            dfsans(nex,chu);

            if(now[j] !=0 )
            {
                nex[0] = now[i] / now[j];
                int a = now[i];
                int b = now[j];
                if( a==now[i] && b==now[j] && (a%b!=0) )
                    dfsans(nex,chu+1);
                else
                    dfsans(nex,chu);
            }
            
        }
    }

    return ;
}

void findans()
{
    vector<double>compute;
    stack<int> ttmp = tmp;

    while (ttmp.size()>0)
    {
        compute.push_back(ttmp.top());
        ttmp.pop();
    }

    aaaa =0 ;
    dfsans(compute,0);
    if(aaaa>0)
    {
        ttmp = tmp;

        struct node ansline;

        while (ttmp.size()>0)
        {
            ansline.line.push_back(ttmp.top());
            ttmp.pop();
        }
        sort(ansline.line.begin(),ansline.line.end());
        ans.push_back(ansline);
    }
}

void dfs(int step)
{
    if(step==0)
    {
        findans();
        return;
    }

    for (size_t i = 1; i <= 13; i++)
    {
        tmp.push(i);
        dfs(step-1);
        tmp.pop();
    }

}

bool cmp(node &a,node &b)
{
    for (size_t i = 0; i < a.line.size(); i++)
    {
        if(a.line[i]<b.line[i]) return true;
        if(a.line[i]>b.line[i]) return false;
    }
    return true;
}

int main()
{
    int n,m,i,j,k,l;

    scanf("%d",&n);
    scanf("%d",&m);
    mm = m;

    dfs(n);


    j=0;
    map<int,bool>maps;
    for (size_t i = 0; i < ans.size(); i++)
    {
        k=0;
        for (size_t j = 0; j < ans[i].line.size(); j++)
        {
            k*=13;
            k+=ans[i].line[j];
        }
        
        if(maps[k] == 0)
        j++;
        maps[k] =1;
    }

    printf("%lu\n",maps.size());
    
    for (size_t i = 0; i < ans.size(); i++)
    {
        k=0;
        for (size_t j = 0; j < ans[i].line.size(); j++)
        {
            k*=13;
            k+=ans[i].line[j];
        }
        if(maps[k] == 0)
        continue;
        maps[k] = 0 ;

        for (size_t j = 0; j < ans[i].line.size(); j++)
        {
            printf("%d ",ans[i].line[j]);
        }

        printf("\n");
    }
    

    return 0;
}```