【Educational Codeforces Round 36 C】 Permute Digits

#include <bits/stdc++.h>
#define ll long long
using namespace std;

const int N = 20;

string s1,s2;
int len1,len2;
int bo[N],a[N],b[N];
vector<int> v,vans;

bool get_ans(){
    for (int i = 0;i <(int) vans.size();i++)
        v[i+1] = vans[i];

    int nn = (int) vans.size();
    for (int i = 0;i<=9;i++)
        for (int j = 1;j <= bo[i];j++)
            v[++nn] = i;

    for (int i = 1;i <= len1;i++)
        if (v[i]!=b[i] && v[i]>b[i]) return false;
        else if (v[i]!=b[i] && v[i]<b[i])return true;
    return true;
}

int main(){
    #ifdef LOCAL_DEFINE
        freopen("rush_in.txt", "r", stdin);
    #endif
    ios::sync_with_stdio(0),cin.tie(0);
    cin >> s1 >> s2;
    len1 = s1.size(),len2 = s2.size();

    if (len1<len2){
        sort(s1.begin(),s1.end());
        reverse(s1.begin(),s1.end());
        cout <<s1<<endl;
        return 0;
    }

    v.resize(len1+1);
    //len1==len2;
    for (int i = 0;i < len1;i++)
        a[i+1] = s1[i]-'0';
    for (int i = 0;i < len2;i++)
        b[i+1] = s2[i]-'0';

    for (int i = 1;i <= len1;i++) bo[a[i]]++;

    bool haved = 0;

    for (int i = 0;i < len1;i++){
        for (int j = 9;j>=0;j--)
            if (bo[j]){
                int k = s2[i]-'0';
                if (j<=k || haved){
                    bo[j]--;
                    vans.push_back(j);
                    if (get_ans()){
                        if (j<k) haved =1;
                        break;
                    }
                    vans.pop_back();
                    bo[j]++;
                }
            }
    }

    for (int i = 0; i< (int) vans.size();i++)
        cout <<vans[i];

    //从大到小枚举第i(1..len1)位
    //剩余的数字从小到大排序。
    //看看组成的数字是不是小于等于b
    //如果是的话。
    //说明第i位就是选这个数字了。
    //接下来枚举下一位。
    return 0;
}