7-3 Path to Infinity（还没ac)

留坑

#include<bits/stdc++.h>
using namespace std;
const int maxn=;
const int mod=;
typedef long long ll;
string s,t;
int tol1=,tol2=,tol3=;
int a[maxn];
string tmp,path;
unordered_set<string> st;
typedef long long ll;
int N,M;
int tle=;
void dfs (int num,int sum) {
    //num表示当前已经用的数字数，sum表示当前表达式的值
    if (num==N) {
        if (sum==M) {
            if (st.size()==) path=tmp;
            st.insert(tmp);
        }
        return;
    }
    if (tol3) {
        for (int i=;i<;i++) {
            if (!a[i]) continue;
            tol3--;
            a[i]--;
            tmp.push_back('*');tmp.push_back(i+'');
            dfs(num+,sum*i);
            tol3++;
            tmp.pop_back();tmp.pop_back();
            a[i]++;
        }
    }
    if (tol1) {
        for (int i=;i<;i++) {
            if (!a[i]) continue;
            tol1--;
            a[i]--;
            tmp.push_back('+');tmp.push_back(i+'');
            dfs(num+,sum+i);
            tol1++;
            tmp.pop_back();tmp.pop_back();
            a[i]++;
        }
    }
    if (tol2) {
        for (int i=;i<;i++) {
            if (!a[i]) continue;
            tol2--;
            a[i]--;
            tmp.push_back('-');tmp.push_back(i+'');
            dfs(num+,sum-i);
            tol2++;
            tmp.pop_back();tmp.pop_back();
            a[i]++;
        }
    }
}
int main () {
    scanf("%d %d",&N,&M);
    cin>>s>>t;
    for (int i=;i<t.length();i++) {
        if (t[i]=='+') tol1++;
        if (t[i]=='-') tol2++;
        if (t[i]=='*') tol3++;
    }
    for (int i=;i<s.length();i++) {
        a[s[i]-'']++;
    }
    for (int i=;i<;i++) {
        if (a[i]) {
            a[i]--;
            tmp.push_back(i+'');
            dfs(,i);
            a[i]++;
            tmp.pop_back();
        }
    }
    printf ("%d\n",st.size());
    if (!st.size()) printf ("Impossible");
    else cout<<path;
    return ;
}