一道枚举题。
暴力枚举数字位数、首位、等差数列的公差即可。注意公差的枚举范围，并且需要看看末尾合不合法。顺便提一下，我是用字符串存储枚举的数字的，所以写了一个 check 函数代替大于号。
Code
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
ll x;
ll top_num(ll x) { ll res; while (x) res = x % 10, x /= 10; return res; }
ll len(ll x) { ll res = 0; while (x) res++, x /= 10; return res; }
bool check(string a, string b) {
if (a.size() > b.size()) return 1;
if (a.size() < b.size()) return 0;
for (int i = 0; i < a.size(); i++) {
if (a[i] - '0' > b[i] - '0') return 1;
if (a[i] - '0' < b[i] - '0') return 0;
}
return 1;
}
int main() {
ios :: sync_with_stdio(0);
cin >> x;
for (ll i = len(x); i; i++) {
for (ll j = top_num(x); j <= 9; j++) {
for (ll d = -10; d <= 10; d++) {
string s = to_string(j);
if (j + (i - 1) * d >= 0 and j + (i - 1) * d <= 9) {
for (ll k = 1; k < i; k++) s += to_string(j + k * d);
}
if (check(s, to_string(x))) {
cout << s;
return 0;
}
}
}
}
return 0;
}