P1143 自己测试样例数据基本是能过的 但只有20分有大佬能来帮助一下我吗谢谢

#include<bits/stdc++.h>
#define int long long
#define syl 35
using namespace std;
const int w[16]={'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
stack<char> st;
int n,m;
string str;
int sum;
int y=1;
signed main()
{
	cin>>n;
	cin>>str;
	cin>>m;
	
	for(int i=0;i<str.size();i++)
	{
		if(str[i]>='A')
			sum=sum+y*(str[i]-'A'+10);
		else
			sum=sum+y*(str[i]-'0');
		y*=n;
	}
//	cout<<sum<<'\n';
	while(sum!=0)
	{
		st.push(w[sum%m]);
		sum/=m;
	}
	while(!st.empty())
	{
		cout<<st.top();
		st.pop();
	}
	return 0;
}