ARC129C

Sol

不难想到可以答案可以为如下形式：777...777a777...77b....777...777，其中字母表示数字。注意到段数不会太多，那么直接包搜间隔的数字即可。

Code

#include <bits/stdc++.h>
#define x first
#define y second
#define pb push_back
#define eb emplace_back
#define pf push_front
#define desktop "C:\\Users\\incra\\Desktop\\"
#define IOS ios :: sync_with_stdio (false),cin.tie (0),cout.tie (0)
#define debug(x) cerr << #x << ' ' << x << endl
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair <int,int> PII;
const int dx[] = {1,0,-1,0},dy[] = {0,-1,0,1};
template <typename T1,typename T2> bool tomax (T1 &x,T2 y) {if (y > x) return x = y,true;return false;
}
template <typename T1,typename T2> bool tomin (T1 &x,T2 y) {if (y < x) return x = y,true;return false;
}
LL power (LL a,LL b,LL p) {LL ans = 1;while (b) {if (b & 1) ans = ans * a % p;a = a * a % p;b >>= 1;}return ans;
}
int fastio = (IOS,0);
#define endl '\n'
#define puts(s) cout << (s) << endl
int n;
vector <string> v;
bool check (string s) {int cnt = 0;for (int i = 0;i < s.size ();i++) {int sum = 0;for (int j = i;j < s.size ();j++) {sum = (sum * 10 + s[j] - '0') % 7;cnt += !sum;}}return cnt == n;
}
void DFS (int u,string ans) {if (u == v.size () - 1) {ans += v.back ();if (check (ans)) {cout << ans << endl;exit (0);}return ;}for (int i = 1;i <= 9;i++) DFS (u + 1,ans + v[u] + (char)('0' + i));
}
void mian () {cin >> n;int x = n;for (int i = 2e3;i >= 1;i--) {string tmp;for (int j = 1;j <= i;j++) tmp += '7';while (i * (i + 1) / 2 <= x) {x -= i * (i + 1) / 2;v.pb (tmp);}}DFS (0,"");puts ("-1");
}
int main () {int T = 1;// cin >> T;while (T--) mian ();return 0;
}