codeforces1373E. Sum of Digits

题意：定义f(x)为x的所有数位和，给定n(0=

k<=9,可得最多一次进位，根据贪心容易知道满足题意的x必定为xx999x的形式，即从十位开始有若干个9，可以枚举个位数c和中间9的个数得到结果

#include 
int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int t;
	std::cin >> t;
	while (t--) {
		int n, k;
		std::cin >> n >> k;
		long long ans = -1;
		for (int c = 0; c < 10; ++c) {
			for (int d = 0; 9 * d + c <= n; ++d) {
				int a = k + 1, b = 0;
				for (int i = 0; i <= k; ++i)
					b += (i + c) % 10 + (i + c < 10 ? 9 * d : 1);
				b = n - b;
				if (b < 0 || b % a != 0)
					continue;
				int x = b / a;
				long long v;
				if (x < 8) {
					v = x;
				}
				else {
					x -= 8;
					v = x % 9;
					x -= v;
					for (int i = 0; i < x / 9; ++i)
						v = 10 * v + 9;
					v = 10 * v + 8;
				}
				for (int i = 0; i < d; ++i)
					v = 10 * v + 9;
				v = 10 * v + c;
				if (v < 1e18 && (ans == -1 || v < ans))
					ans = v;
			}
		}
		std::cout << ans << "\n";
	}
	return 0;
}