2016 World Final E Forever Young

题意：

。。

思路：

xjb搞一下就行了。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <algorithm>
#include <ctime>
#pragma comment(linker,"/STACK:102400000,102400000")
using namespace std;

#define eps 1e-10
#define N 200020
#define B 20
#define M 3000020
#define inf 0x3f3f3f3f
#define LL long long
#define pii pair<int, int>
#define MP make_pair
#define fi first
#define se second
#define mod 1000000007

#define md (ll + rr >> 1)

LL n, l;
LL ans;
int a
;

void check(LL b) {
if(b < 10) return;
LL nn = n;
int len = 0;
while(nn) {
LL t = nn % b;
if(t > 9) return;
nn /= b;
a[len++] = t;
}
reverse(a, a + len);
LL tmp = 0;
for(int i = 0; i < len; ++i) tmp = tmp * 10 + a[i];
if(tmp < l) return;
if(ans == -1 || ans < b) ans = b;
}

void solve(LL a, LL b, LL c, LL &x) {
LL l = 1, r = sqrt(c + 0.5);
while(l < r) {
LL mid = (l + r) / 2;
if(1.0 * mid * (a * mid + b) > 5e18) r = mid;
else if(mid * (a * mid + b) < c) l = mid + 1;
else r = mid;
}
if(l * (a * l + b) == c) x = l;
else x = -1;
}

int main() {
ans = -1;
cin >> n >> l;
for(int x = 1; x <= 9; ++x) {
for(int y = 0; y <= 9; ++y) {
if((n - y) % x) continue;
LL b = (n - y) / x;
if(b < 10) continue;
check(b);
}
}
for(int a = 1; a <= 9; ++a) {
for(int b = 0; b <= 9; ++b) {
for(int c = 0; c <= 9; ++c) {
LL x1;
solve(a, b, n - c, x1);
check(x1);
}
}
}
for(int i = 10; 1LL * i * i * i <= n; ++i) check(i);
cout << ans << endl;
return 0;
}