十进制转十六进制, 附上我的代码

$n \le 10^{17}$

#include <bits/stdc++.h>
using namespace std;
#define ULL unsigned long long int
#define cst 11111
char c[cst];
int cnt = 0;
int main() {
	ULL a;
	cin >> a;
	while(a >= 0) {
		if(a % 16 < 10) {
			c[++cnt] = int(a % 16) + '0';
		} else {
			c[++cnt] = (int(a % 16) - 10) + 'A';
		}
		a /= 16;
	}
	for(int i = cnt; i >= 1; i--)
		cout << c[i];
	return 0;
}