Compare Version Numbers leetcode

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 = 0, j = 0;
    while (i < version1.length() || j < version2.length())
    {
        int a1 = 0;
        int a2 = 0;        
        while (i < version1.length() && version1[i] != '.')
            a1 = a1 * 10 + version1[i++] - '0';
        while (j < version2.length() && version2[j] != '.')
            a2 = a2 * 10 + version2[j++] - '0';
        i++; j++;
        if (a1 > a2)
            return 1;
        else if (a1 < a2)
            return -1;
    }
    return 0;
}