CareerCup

for this simple questions, make sure all border cases, overflows, etc. are covered.
as well, maybe define the allowed token/string exactly (e.g. using regexp)
that would be "\s*0x[0-9a-zA-Z]+\s*"

#include <string>
#include <iostream>
#include <locale>

using namespace std;

int hexCharToInt(char c)
{
	if (c >= '0' && c <= '9') return c - '0';
	if (c >= 'A' && c <= 'F') return 10 + c - 'A';
	if (c >= 'a' && c <= 'f') return 10 + c - 'a';
	throw "unexpected character";
}

unsigned long hex2int(const string& str) 
{
	size_t i = 0;
	while (i < str.size() && isspace(str[i])) i++; // trailing spaces
	if (i == str.size()) throw "unexpected end of string"; // string must not consist of only white-spaces
	if (str[i++] != '0') throw "unexpected character"; // a 0x must initiate the hex string
	if (i == str.size()) throw "unexpected end of string";  // 0 is not a valid hex string
	if (str[i++] != 'x') throw "unexpected character"; // 0x to initiate hex string
	if (i == str.size()) throw "unexpected end of string"; // string must not end now
	if (isspace(str[i])) throw "unexpected character"; // no space allowed after "0x"
	unsigned long value = 0;
	while(i < str.size() && !isspace(str[i])) {
		unsigned long or = value;
		value <<= 4;
		if (value >> 4 != or) throw "overflow";
		value |= hexCharToInt(str[i]);
		i++;
	}
	while (i < str.size() && isspace(str[i])) i++; // trailing spaces, if any
	if (i < str.size() - 1) throw "unexpected character"; // no other characters expected
	return value;
}


int main()
{
	cout << hex2int("0x0") << endl;
	cout << hex2int("  0x80") << endl;
	cout << hex2int("  0xff  ") << endl;
	cout << hex2int("  0xffffffff  ") << endl;
	cout << hex2int("  0x1fffffff1  ") << endl; // should throw overflow exception
	cout << hex2int("  1fffffff1  ") << endl; // should throw unexpected char exception
}

//java8

int convertFromHex(char[] s) {
        int digit, result = 0, i = (s[0] == '0') && (s[1] == 'x' || s[1] == 'X') ? 2 : 0;
        for (; i < s.length; i++) {
            char c = s[i];
            if (c >= '0' && c <= '9') {
                digit = c - '0';
            } else if (c >= 'A' && c <= 'F') {
                digit = 10 + c - 'A';
            } else {
                throw new IllegalArgumentException("not hex");
            }

            try {
                result =
                        Math.addExact(Math.multiplyExact(16, result),
                                digit);
            } catch (Exception e) {
                throw new IllegalArgumentException("overflow", e);
            }

        }
        return result;
    }

public class HexToDecimal {

    public static void main(String[] args) {
        System.out.println(convertFromHex("36F9BFB3"));
    }

    static int convertFromHex(String str) {
        char[] s = str.toUpperCase().toCharArray();
        int digit, result = 0, i = (s[0] == '0') && s[1] == 'X' ? 2 : 0;
        for (; i < s.length; i++) {
            char c = s[i];
            if (c >= '0' && c <= '9') {
                digit = c - '0';
            } else if (c >= 'A' && c <= 'F') {
                digit = 10 + c - 'A';
            } else {
                throw new IllegalArgumentException("not hex");
            }

            try {
                result =
                        Math.addExact(Math.multiplyExact(16, result),
                                digit);
            } catch (Exception e) {
                throw new IllegalArgumentException("overflow", e);
            }

        }
        return result;
    }
}