poj2190,

## Problem:

ISBN
 Time Limit: 1000MS Memory Limit: 65536K Total Submissions: 11815 Accepted: 4078
Description
Farmer John's cows enjoy reading books, and FJ has discovered that his cows produce more milk when they read books of a somewhat intellectual nature. He decides to update the barn library to replace all of the cheap romance novels with textbooks on algorithms and mathematics. Unfortunately, a shipment of these new books has fallen in the mud and their ISBN numbers are now hard to read.

An ISBN (International Standard Book Number) is a ten digit code that uniquely identifies a book. The first nine digits represent the book and the last digit is used to make sure the ISBN is correct. To verify that an ISBN number is correct, you calculate a sum that is 10 times the first digit plus 9 times the second digit plus 8 times the third digit ... all the way until you add 1 times the last digit. If the final number leaves no remainder when divided by 11, the code is a valid ISBN.

For example 0201103311 is a valid ISBN, since
10*0 + 9*2 + 8*0 + 7*1 + 6*1 + 5*0 + 4*3 + 3*3 + 2*1 + 1*1 = 55.

Each of the first nine digits can take a value between 0 and 9. Sometimes it is necessary to make the last digit equal to ten; this is done by writing the last digit as X. For example, 156881111X is a valid ISBN number.

Your task is to fill in the missing digit from a given ISBN number where the missing digit is represented as '?'.
Input
* Line 1: A single line with a ten digit ISBN number that contains '?' in a single position
Output
* Line 1: The missing digit (0..9 or X). Output -1 if there is no acceptable digit for the position marked '?' that gives a valid ISBN.
Sample Input
`15688?111X`
Sample Output
`1`
Source
USACO 2003 Fall Orange

## Solution:

Basically, it's a very easy simulation.
PS:
1. if the unknown number is 10, you need to use 'X' to represent it.
2. the 'X' can only exist as the last digit.

## Source Code:

//Wed Apr 14 13:57:18 CDT 2010
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>

using namespace std;

int main(int argc, const char* argv[])
{
freopen("input.in", "r", stdin);
freopen("output.out", "w", stdout);
string str;
while (cin >> str)
{
int idx;
int sum = 0;
for (int i = 0; i < (int) str.size(); i++)
{
if (isdigit(str[i]))
sum += (10 - i) * (str[i] - '0');
else if (str[i] == 'X')
sum += (10 - i) * 10;
else
{
idx = i;
}
}
sum %= 11;
bool flag = false;
for (int i = 0; i < 10; i++)
{
if ((sum + (10 - idx) * i) % 11 == 0)
{
cout << i << endl;
flag = true;
break;
}
}
if((sum + (10 - idx) * 10) % 11 == 0 && idx == 9)
{
cout << "X" << endl;
flag = true;
}
if(flag == false)
cout << -1 << endl;
}
fclose(stdin);
fclose(stdout);
return 0;
}