Compare two version numbers version1 and version2.
If version1 > version2 return 1, if version1 < version2 return -1, otherwise return 0.

You may assume that the version strings are non-empty and contain only digits and the . character.
The . character does not represent a decimal point and is used to separate number sequences.
For instance, 2.5 is not "two and a half" or "half way to version three", it is the fifth second-level revision of the second first-level revision.

Here is an example of version numbers ordering:

0.1 < 1.1 < 1.2 < 13.37

自己写的代码跟discuss的最优代码几乎相同，不知道还有没有优化空间

int compareVersion(string version1, string version2) {

    int i = , j = ;

    while (i < version1.length() || j < version2.length())

    {

        int a1 = ;

        int a2 = ;

        while (i < version1.length() && version1[i] != '.')

            a1 = a1 *  + version1[i++] - '';

        while (j < version2.length() && version2[j] != '.')

            a2 = a2 *  + version2[j++] - '';

        i++; j++;

        if (a1 > a2)

            return ;

        else if (a1 < a2)

            return -;

    }

    return ;

}