Sixty-four. Conversion between decimal and hexadecimal

Decimal to Hexadecimal

#include<iostream>
using namespace std;
int main()
{
    
    
    int a;
    cin >> a;
    int i = 0;
    char b[100];
    if(a==0)
    {
    
    
    	cout << "0";
	}
	else
	{
    
    
		while(a!=0)
		{
    
    
			if(a % 16 >= 10)
			{
    
    
				b[i++] = a % 16 + 'A' - 10;
			}
			else
			{
    
    
				b[i++] = a % 16 + '0';
			}
			a /= 16;
		}
	}
	for(int k = i-1; k>=0; k--)
	{
    
    
		cout << b[k];
	}
	return 0;
} 

Hexadecimal to Decimal

#include<iostream>
using namespace std;
int main()
{
    
    
	string s;
	while(cin>>s)
	{
    
    
		int len = s.length();
		int sum = 0;
		for(int i = 0; i<len ; i++)
		{
    
    
			if(s[i]>='A' && s[i]<='F')
			{
    
    
				sum = sum * 16 + s[i] - 'A' + 10;
			}
			else
			{
    
    
				sum = sum * 16 + s[i] + '0';
			}
		}
		cout << sum;
	}
	return 0;
}