天长地久

https://www.acwing.com/problem/content/description/4275/

优质题解：https://www.acwing.com/solution/content/89003/

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
typedef pair<int,int> PII;
int m,k,cnt = 0,f,c,n;
int a[10];

void dfs(int x,int sum,vector<PII>& v)
{   
    //if((k - x ) * 9 + sum < m) return ; //后面的位不足以组合 
    if(sum > m || x > k) return ;
    if(x == k && sum == m  && a[cnt - 1] != 0 && a[0] != 9)//第一位不能是0最后一位不能是9
    {    
        int res = 0;
        for(int i = cnt - 1; i >= 0 ; i --) res = res * 10 + a[i];
        for(int i = 1; i <= c; i ++) res = res * 10 + 9;
        v.push_back({n,res}),f = 1;
        return ;
    }
    for(int i = 0; i <= 9; i ++)
    {   
        a[cnt ++] = i;
        dfs(x + 1,sum + i,v);
        a[cnt--]=0;
    }
}
int main()
{
    int j;
    cin >> j;
    for(int i = 1; i <= j; i ++)
    {
        f = 0;
        cin >> k >> m ;
        cout << "Case " << i << '\n';
        int x = m; vector<int>p;
        for(int i = 2; i <= x / i; i ++) //分解质因数
        {
            if(x % i == 0)
            {   
                p.push_back(i);
                while(x % i == 0) x /= i;
            }
        }
        if(x > 1) p.push_back(x);
        if(p.back() <= 2) f = 0; //不存在大于2的质因子
        else{
            vector<PII>v;
            for(auto it : p)
            {   
                if(it <= 2) continue;
                n = it;
                for(int i = 1; n < m; i ++)
                {    
                    n = it * i;
                    if((m - n + 1) % 9 == 0)
                    {
                        c = (m - n + 1) / 9; //末尾有c位9
                        dfs(c,c * 9,v);
                    }
                }
            }
        sort(v.begin(),v.end());
        for(int i = 0; i < v.size(); i ++)
        {
            if(i != 0) puts("");
            cout << v[i].first << ' ' << v[i].second;
        }
        }
        if(f == 0) cout << "No Solution";
        if(i != j) puts("");
    }
    return 0;
}